(CNN) Russian forces have capabilities in place along the Ukraine border to carry out a swift and immediate invasion, including erecting supply lines such as medical units and fuel that could sustain a drawn-out conflict, should Moscow choose to invade, two sources familiar with the latest intelligence assessments told CNN.
The new details about the Russian buildup underscore US officials' heightened alarm over the movements. The current levels of equipment stationed in the area could supply front-line forces for seven to 10 days and other support units for as long as a month, according to one source familiar with the matter.
Democratic Rep. Mike Quigley of Illinois, who sits on the House Intelligence Committee, told CNN that he believes Russia is positioned to invade "when they want," adding that "Russia's capabilities would be equivalent to a modern-day blitzkrieg."
A senior administration official told CNN that the US has "seen additional Russia troops added to the border region in recent days," but the source declined to detail how many troops.
It is still unclear whether Russian President Vladimir Putin intends to invade or whether the buildup is a game of brinkmanship to get concessions from the West, officials said. But the apparent military readiness has raised alarms among Western officials, who say Russia is positioning itself to be able to launch an attack on very short notice.
"We don't know whether President Putin has made the decision to invade. We do know that he is putting in place the capacity to do so on short order should he so decide," US Secretary of State Antony Blinken told reporters in Latvia on Wednesday. "We must prepare for all contingencies."
A senior Western intelligence official similarly said Russia is trying to "set the stage to be ready to conduct a fairly large-scale operation" into Ukraine in an effort to coerce Kiev away from its drift into the Western European and NATO orbit.
Russia has denied in recent days that it has any plans to attack Ukraine but has also demanded security guarantees from the West, such as a pledge that it won't allow Ukraine to join NATO.
"So if NATO still refuses to discuss this theme or the guarantees or ideas put forward by the President of Russia, Vladimir Putin, of course we will take measures to ensure that our security, our sovereignty and our territorial integrity does not depend on anyone else," Russian Foreign Minister Sergey Lavrov said Thursday.
The US is still deliberating what kind of military aide to send to Ukraine in the face of the Russian buildup.
Ukraine will commission new military patrol boats, supplied by the US, in the coming days to bolster its missions in the Black Sea, according to two people familiar with the matter. The US is planning to send more anti-tank weapons like Javelin missiles, but has for now backed off of providing Ukraine with surface-to-air missiles, like Stingers, which Russia would view as provocative, sources said.
The US and its allies in Europe and NATO have warned Russia of severe economic sanctions should it move to invade Ukraine.
But Western officials are reluctant to get drawn into a direct military conflict with Russia, which Moscow understands, the senior Western intelligence official said.
"If you threaten force, and you keep your force in the field, and the interventions from the West, politically and militarily, are mild or nonexistent, that emboldens you," that official said. "Generally speaking, I think the aim here is to make clear, 'I'm willing to take this big risk and use force and I'm betting that you are unwilling to engage with me and have a loss of life.' "
Western officials have been pushing for a diplomatic solution they hope will de-escalate tensions -- specifically, the full implementation of the Minsk agreements, a ceasefire protocol signed by Ukraine and Russia in 2015.
"It takes two to tango, and if ... our Russian friends are prepared to implement their commitments under Minsk and our Ukrainian friends are as well, we will fully support that, and that is the best way to avert a renewed crisis in Ukraine," Blinken said on Thursday.
Critics have characterized the Minsk agreements as disproportionately advantageous to Moscow. But the US has been pushing Kiev and Moscow to implement their terms, including constitutional changes by Ukraine that would give autonomy to the separatist-run eastern Ukrainian city of Donetsk. The city has been a major battleground between Ukrainian and pro-Russian forces for more than seven years, and some Biden administration officials believe that if Ukraine were to act first on Minsk, then the onus would be on Russia to de-escalate.
But Ukrainian officials have pushed back on such a request, arguing that the Minsk agreements do not require such constitutional changes, people familiar with the discussions said. Russia also has yet to implement several of the Minsk provisions, including a ceasefire, withdrawal of all heavy weapons from the line of contact in eastern Ukraine and an all-for-all political prisoner exchange, as Blinken outlined on Thursday.
"And just as we've been clear with Moscow, we're also urging Ukraine to continue to exercise restraint," Blinken said. "Because again, the Russian playbook is to claim provocation for something that they were planning to do all along."