(CNN) US Secretary of State Antony Blinken said Wednesday that the United States is "concerned by reports of unusual Russian military activity" and the possibility that Russia may be "attempting to rehash" its 2014 invasion of Ukraine.
Blinken's comments came a week after Russia's powerful security chief did not deny that Moscow was moving troops or assuage the US' concerns about Russia's intentions during a meeting with CIA director Bill Burns, according to four people briefed on the discussion.
Speaking alongside Ukrainian Foreign Minister Dmytro Kuleba at the State Department, Blinken said that the US is "concerned by reports of unusual Russian military activity," and is "monitoring very closely" the Russia activity.
"Our concern is that Russia may make a serious mistake of attempting to rehash what it undertook back in 2014, when it amassed forces along the border, crossed into sovereign Ukrainian territory and did so claiming falsely that it was provoked," Blinken said, referring to Russia's invasion of Crimea. "So the playbook that we've seen in the past was to claim some provocation as a rationale for doing what it, what it intended and planned to do. All which is why we're looking at this very carefully."
The top US diplomat also reiterated the US' commitment to Ukraine's sovereignty and independence, calling it "ironclad."
In a post on his Facebook page Wednesday, Ukrainian President Volodymyr Zelensky thanked the US for its support and for intelligence it had shared about the situation.
"Our Western partners have provided data on the active movement of Russian troops along the Ukrainian border and the increase in their concentration," he wrote. "Foremost, we are very grateful to our partners for this information. This is a proof of support of Ukraine."
He echoed Blinken's remarks about Russia's playbook, saying that "from [the] Russian side, we hear accusations that it is Ukraine that is delaying the peace process. ... I hope now the whole world can clearly see who really wants peace and who is concentrating almost 100,000 troops on our border."
Kuleba also said the US and Ukraine shared "elements" with each other Wednesday regarding the Russian military activities. "What we heard and saw today in Washington, DC, corresponds to our own findings and analysis, adds some new elements, which allow us to get a better and more comprehensive picture," he said.
Blinken and Kuleba addressed reporters at the conclusion of a strategic dialogue that led to the signing of a renewed Ukraine-US Strategic Partnership. The Ukrainian official expressed gratitude to the US for deepening defense and security cooperation "to help Ukraine build its capacity to defend itself and also to deter Russia to demotivate them from taking further aggressive actions."
"The best way to deter aggressive Russias is to make it clear for the Kremlin that Ukraine is strong," Kuleba said. "I have repeated on numerous occasions that Russian aggression against Ukraine will end on the day Ukraine's place as part of the West is institutionalized and undoubted. Today, we have made another important step in that direction."
Blinken said the US would continue to "consult closely as well with allies and partners" about Russia's troop movements. On Friday, the US sent out a formal diplomatic note, known as a démarche, to NATO allies providing them with additional intelligence and requesting further coordination in response to the irregular troop movements, a person familiar with the message said.
"As we make clear, any escalatory or aggressive actions will be of great concern to the United States," Blinken said Wednesday. He added that the US will continue to support deescalation in the region and a diplomatic resolution to the conflict in eastern Ukraine.
Kuleba indicated that finding a diplomatic way out could be difficult. Pointing to various Russian efforts to destabilize Europe -- including its coercive use of energy supplies, "propaganda efforts, disinformation, cyberattacks, military buildups, an attempt of Russia to digest Belarus" -- the Ukrainian diplomat warned that "in this complicated game, we have to remain vigilant, we have, have to be resilient."
The joint press appearance is just the latest effort by the Biden administration to demonstrate support for Ukraine, some of which has happened behind closed doors.
Burns met with Russian President Vladimir Putin and Russia's security chief, Nikolai Patrushev, last week, as the Biden administration has grown increasingly concerned about Russia's recent troop movements near Ukraine. As CNN first reported Friday, President Joe Biden dispatched Burns to Moscow to deliver a clear message to the Kremlin that the US is monitoring the movements closely.
In response, Patrushev did not deny that Russia was accumulating military units in the area, according to the four people briefed on the discussion. That stood in contrast to the Kremlin's recent assertion that satellite images showing such a buildup -- including self-propelled guns, battle tanks and infantry fighting vehicles at a training ground roughly 186 miles from the border -- were "low-quality" and "fake."
The exact number of new Russian forces currently staged there is not clear. One official told CNN that the number of units in the area has increased by about two-thirds in recent weeks.
But US officials believe that the military activity in western Russia is irregular, multiple sources told CNN. For one thing, it is the off-season for military exercises in Russia. Some of the units on the ground are also a long way from their headquarters, said Michael Kofman, an expert on Russian military studies at the Center for Naval Analyses.
"The big question is why are units from the Central Military District 3,000 miles from their garrison hanging around for the winter?" he said. "It is difficult to find an innocuous explanation for the military activity that's been taking place."
Russian special forces, known as the Spetsnaz, are also on the ground there, two people familiar with the intelligence said. Special forces greatly expand hybrid warfare capabilities, such as psychological operations.
Still, US officials and outside experts closely tracking the movements say they don't see signs that Russia is preparing to invade in the coming days or weeks. They don't yet see what is known as a "logistics tail," for example -- the supply lines that would be needed for any military to launch a combat operation.
Those sources say Russia does appear to be positioning itself to be able to launch a military campaign in the coming months, perhaps as soon as January. Missing ingredients, like a logistics tail, could be amassed on short notice, Kofman noted.
Following his meetings in Russia, Burns spoke to Zelensky by phone in an attempt to diffuse tensions, the sources told CNN. A senior State Department official was also dispatched to Kiev on Thursday to support those efforts.
The flurry of high-level diplomacy underscores how seriously the Biden administration is taking the latest Russian troop movements, even after an earlier buildup this spring ultimately did not lead to a renewed invasion. Tensions between Ukraine and Russia have also been exacerbated in recent weeks by a deepening Ukrainian energy crisis that Kiev believes Moscow has purposefully provoked.
"The buildup, coupled with the energy blackmail, does suggest a more aggressive Russian posture," an adviser to Zelensky told CNN.
Pentagon press secretary John Kirby said on Friday that the "scale" and "the size of the units that we're seeing" from Russia is "unusual."
"We continue to monitor this closely, and as I've said before, any escalatory or aggressive actions by Russia would be of great concern to the United States," he said.
This story has been updated with additional reporting.