Washington(CNN) As North Korea continues its march towards developing a reliable long-range nuclear missile, US officials are becoming increasingly vocal about concerns over Pyongyang's ties to another familiar adversary: Iran.
Despite current restrictions in place to monitor and curtail Iran's nuclear program, several lawmakers and members of the intelligence community have warned in recent weeks that Tehran could theoretically purchase technology or knowledge related to building a nuclear weapon in the future.
The ominous predictions have coincided with escalating tensions between the US and North Korea: Pyongyang conducted its sixth nuclear test and issued a variety of heated threats, including a retaliatory threat to launch missiles near the American territory of Guam.
North Korea and Iran have collaborated on missile development in the past, and the State Department is currently monitoring weapons transactions and attempting to determine whether there has been cooperation between the two nations on ballistic missile capability which does not fall under the restrictions agreed to in the 2015 Iran nuclear deal, according to acting Assistant Secretary of State Susan Thornton, Bureau of East Asian and Pacific Affairs.
But little to no hard evidence has been presented to suggest that the Iranians are currently working with Pyongyang to enhance their nuclear program, and intelligence suggests North Korea is still addressing issues with its own efforts.
Last week, Japan asked Iran to cooperate in international efforts against North Korea's nuclear program. Iranian President Hassan Rouhani has also said that he does not want a nuclear North Korea.
So what is prompting some US officials to sound the alarm?
It is no secret that North Korea's nuclear and missile programs have made major strides in recent months, and its weapons tests amid escalating tensions with the US have prompted global condemnation and increased sanctions on the rogue nation.
The standoff between North Korea and the US has raised myriad complex challenges for military and intelligence officials tasked not only with addressing the immediate threat of potential escalation but also preparing for scenarios that could emerge as Pyongyang continues to improve its capability.
Preventing North Korean leader Kim Jong Un from achieving his nuclear ambitions remains a top priority for the Trump administration -- but should those efforts fail, some US officials warn that the implications could stretch beyond dealing with a nuclear-armed North Korea.
On Monday, CIA Director Mike Pompeo said it is "fair to say" that as North Korea improves its capabilities, it could be willing to share that knowledge and technology if approached by potential customers -- namely, Iran.
"The North Koreans have a long history of being proliferators and sharing their knowledge, their technology, their capacities around the world," Pompeo said in a Fox News interview.
"As North Korea continues to improve its ability to do longer-range missiles and to put nuclear weapons on those missiles, it is very unlikely, if they get that capability, that they wouldn't share it with lots of folks, and Iran would certainly be someone who would be willing to pay them for it," he added.
Pompeo's comments were notably conditional as he addressed the issue of proliferation -- an issue that has been a long-standing concern for US intelligence officials.
But his remarks also reflect a recent effort by some US officials and lawmakers to publicly highlight the potential link between North Korea and Iran that also coincides with recent high-level diplomatic activity between the two nations.
On Tuesday, Thornton said the State Department continues to track illicit arms shipments from North Korea to both Iran and Syria, noting "there are some transactions we are worried about," when pressed on the issue by Texas Republican Rep. Michael McCaul.
"We do track any kind of illicit proliferation networks from the North Koreans and go after those transactions, again, with colleagues at Treasury and other agencies," Thornton told lawmakers on the House Foreign Affairs Committee.
"When we find them, we try to block them or deter them, and we've had some success. It's a continuing effort on our part, and we devote a lot of attention to that," she said.
Thornton said the US is also monitoring collaboration between Tehran and North Korea on ballistic missiles but did not indicate that the relationship directly involves nuclear proliferation.
The United Nations panel on North Korean sanctions is also investigating "reported prohibited chemical, ballistic missile and conventional arms cooperation" between North Korea and Syria, according to its latest assessment.
These findings are also fueling concerns that if North Korea will sell to one malign actor in the Middle East, it could just as easily sell to a country like Iran.
In July, Vice Chairman of the Joint Chiefs of Staff Gen. Paul Selva said he agreed with assessments by the intelligence community that North Korea could become a nuclear arms proliferator.
"There's no evidence that they have engaged in proliferation of their long-range ballistic missile technology, but they have proliferated every other weapons system that they've ever invented. So it's a pretty clear pathway to the potential proliferation of these kinds of weapons systems," Selva told lawmakers on the Senate Armed Services Committee.
US-Iranian tensions seemed to cool after international negotiators reached an agreement aimed at reining in Iran's nuclear program in 2015.
But murmurs linking Tehran and North Korea's nuclear program have only grown louder against the backdrop of President Donald Trump's repeated pledges to take a tough line on Iran -- including his calls on the campaign trail for the US to tear up or renegotiate the agreement, which he has decried as "the worst deal ever."
While Trump has since twice re-certified the deal, most recently in July, he also recently approved new sanctions against six Iran-based satellite companies following a recent Iranian rocket launch -- a move that prompted a bristling response from Tehran.
Trump will face another re-certification deadline in the coming weeks and has again signaled a desire to withdraw from the agreement despite a recent report from the International Atomic Energy Agency that all parties were in compliance.
The State Department and National Security Council did not immediately respond to CNN's request for comment regarding the administration's stance on whether links between North Korea and Iran relate to the upcoming re-certification deadline.
The Trump administration is continuing its review of the Iran nuclear deal but the policy toward Iran should include "the totality of Iranian threats, not just Iran's nuclear capabilities," US Secretary of State Rex Tillerson said Thursday in London.
Speaking at a news conference with his British counterpart, Boris Johnson, Tillerson said the preface to the agreement said its implementation "will positively contribute to regional and international peace and security."
"In our view, Iran is clearly in default of these expectations of the JCPOA through their actions to prop up the Assad regime (in Syria), to engage in malicious activities in the region -- including cyberactivity -- aggressively developing ballistic missiles; and all of this is in defiance of UN Security Council resolution 2231, thereby threatening -- not ensuring, but threatening -- the security of those in the region as well as the United States itself," Tillerson added.
Trump's looming decision to either re-certify or withdraw from the Iran deal has been amplified by the heightened concern over North Korea's nuclear weapons program, but experts offer a range of assessments regarding the potential threat of nuclear or missile cooperation between Pyongyang and Tehran in the future.
North Korea and Iran do have a history of joint missile development dating back to the Iran-Iraq war in the 1980s, and both countries have been linked to Pakistani nuclear physicist and accused proliferator Abdul Qadeer Khan in the past.
Since that time, Iran has made some independent developments in its missile capability, but much of its progress has been coupled with assistance from Pyongyang, according to Bruce Bennett, a senior researcher who specializes in North Korea at the RAND Corporation.
Iran currently possesses more ballistic missiles than any other country in the Middle East but remains dependent on foreign suppliers for missile development and production -- an ongoing challenge that raises questions about whether they can or will develop an ICBM capable of carrying a nuclear missile, according to the Nuclear Threat Initiative, a nonpartisan, Washington-based nonprofit organization.
"We have to anticipate that while focusing on North Korea that it is a potential trigger of great deal of proliferation," Bennett told CNN, adding that the potential for cooperation stems from the shared goal of both countries to mount a nuclear weapon on an intercontinental ballistic missile and North Korea's willingness to act like a "cartel" in its willingness to share information for a price.
The idea that North Korea and Iran's past missile-sharing relationship could eventually evolve to include nuclear weapons is "appropriate as each side has something to offer," according to Anthony Ruggiero, a former official at the US State and Treasury departments.
"Iran and North Korea both have enrichment programs, Pyongyang has an advanced nuclear weapons program, and Tehran has cash," he said, adding that Pyongyang desperately needs hard currency to sustain its strategic programs and its elites.
But although their history of missile cooperation indicates that the two nations could potentially strike a deal over nuclear weapons in the future, some argue that concerns over the link between North Korea and Iran misses a more urgent point.
"Proliferation is part of the problem but not the centerpiece when it comes to North Korea," according to Leon Sigal, director of the Northeast Asia Cooperative Security Project at the Social Science Research Council in New York. Sigal added that the recent emphasis on Pyongyang's links to Iran are the product of an internal argument within the Trump administration over the Iran deal.
Sigal's suggestion that attempts to publicly link Iran and North Korea's nuclear development efforts stems from conflicting views over Trump's desire to withdraw from the Iran nuclear deal.
Trump and several of his top diplomats, including US ambassador to the United Nations Nikki Haley, have repeatedly made the case for abandoning the agreement despite warnings from 80 of the world's leading nonproliferation specialists that doing so would not only isolate the US but could ultimately encourage Iran to resume its nuclear program and "create a second major nonproliferation crisis."
"The US has to find out if North Korea is willing to negotiate, and there are people in the Trump administration willing to get to that point," Sigal said, noting that Pompeo's comments suggest a strong argument to negotiate in hopes of bounding Pyongyang's program before they reach the point of being able to sell fissile material.
If the US doesn't negotiate, then North Korea will ultimately "have an unbounded nuclear program and can sell its secrets or technology to other people," he said, adding that the more urgent problem facing the US is that, despite levying more sanctions, it still hasn't determined whether Pyongyang is willing to stop its programs now and come to the table.
"Negotiations mean putting forth a real proposal that addresses North Korea's security concerns in addition to our own interest, and we are not at that point yet," Sigal said.