U.S. President Joe Biden has said that he is prepared to "make it very, very difficult" for Russia to launch an attack against Ukraine, amid growing Western concerns over Moscow’s intentions in the face of a Russian military buildup near the Ukrainian border.
The warning came as Biden and Russian President Vladimir Putin are expected to hold a video call in the coming days, both sides confirmed on December 3.
Washington and Kyiv say Moscow has amassed tens of thousands of troops near Ukraine's border, and is potentially planning an offensive as early as January.
But U.S. officials have said they were unclear of Russia's motives and whether Putin has made the political-military decision to stage an offensive.
Some analysts say Russia is saber-rattling to extract concessions from the United States and its allies over issues such as NATO's eastward expansion and weapons shipments to Ukraine.
Russia forcefully seized Ukraine's Crimean Peninsula in 2014 and is backing separatists in eastern Ukraine in an ongoing conflict that has claimed more than 13,200 lives over the past seven years.
"What I am doing is putting together what I believe to be the most comprehensive and meaningful set of initiatives to make it very, very difficult for Mr. Putin to go ahead and do what people are worried he's going to do," Biden told reporters.
The president said his administration was in "constant contact" with Ukraine and European allies about the situation, following weeks of reports Russian troops, tanks, and heavy weaponry were massing near the Ukrainian border.
A report in The Washington Post on December 3, citing a U.S. official and an unclassified intelligence document, said Russia could be planning a multifront offensive involving up to 175,000 troops as soon as early next year.
Earlier on December 3, Ukrainian Defense Minister Oleksiy Reznikov told lawmakers that the country's intelligence had assessed that the "likelihood of large-scale escalation by Russia exists."
"The most likely time to reach readiness for escalation will be the end of January," Reznikov said.
While Ukraine has estimated that around 95,000 Russian troops are currently near its borders, the U.S. intelligence assessment put the current number at 70,000 but predicted a potentially higher buildup.
"The plans involve extensive movement of 100 battalion tactical groups with an estimated 175,000 personnel, along with armor, artillery and equipment," an administration official told The Washington Post.
Moscow blames Ukraine and its Western backers for fanning recent tensions, pointing to what it says is a similar Ukrainian military buildup and a failure by Kyiv to meet its commitments under the Minsk agreements aimed at putting an end to the conflict in eastern Ukraine.
Earlier this week, Putin reiterated Russia had "red lines" about any prospective NATO membership for Ukraine, and raised concerns about Western weapon supplies to Kyiv and military drills in the Black Sea.
Washington has rejected Russia's ultimatums about weapon supplies and Ukraine one day joining NATO, but the issue is likely to dominate discussions between Biden and Putin.
The Kremlin said that the Russian leader would seek binding guarantees precluding NATO's expansion to Ukraine during the call with Biden.
"I don't accept anyone's red line," the U.S. president said.
It's unclear what the U.S. response would be to any Russian offensive. At a NATO ministerial in Latvia on December 1, Secretary of State Antony Blinken threatened "a range of high-impact economic measures that we've refrained from using in the past."
He did not specify what sanctions were being weighed, but one potential could be to cut off Russia from the SWIFT system of international payments, a move that would be devastating to the Russian financial system.