Ukrainian soldiers walks at the line of separation from pro-Russian rebels near Katerinivka, Donetsk region, Ukraine, Tuesday, Dec 7, 2021. AP
The Kremlin has denied the plans and blames rising tensions on NATO. Here are answers to key questions:
Is an invasion likely?
Ukraine and its Western allies have accused Moscow of massing tens of thousands of troops near its border with Ukraine in preparation for an invasion.
Ukraine's military command believes a possible escalation could come at the end of January.
Moscow said the troop build-up is part of routine deployments, saying it has the right to do what it wants on its territory.
"Everything Russia does is on the territory of the Russian Federation," Kremlin spokesman Dmitry Peskov said Wednesday.
Experts doubt the likelihood of an offensive, especially since the Ukrainian army has seen significant improvements after more than seven years of conflict against pro-Russia separatists in the east.
A Russian-Ukrainian confrontation would therefore risk a huge human and financial toll.
Moscow's troop surge follows a similar build-up in the Spring, when the first fears of an invasion emerged but never materialised.
Some analysts suggest Russia's aim was to extract diplomatic benefits.
The Kremlin and its army however can act quickly, as they did during Moscow's annexation of Crimea from Ukraine in 2014.
In 2008, the Russian army also crushed Georgia's army -- which wanted to take back the separatist region of South Ossetia -- in five days.
What does Putin want?
Putin has accused the West of ignoring Russia's "red lines" and stresses Kiev is approaching them dangerously by using Turkish military drones and reaffirming its ambitions to join NATO.
During his call with Biden Tuesday, Putin demanded "legal guarantees" that would exclude NATO expanding into Ukraine.
The longtime Russian leader accuses the alliance -- set up to counter the Soviet Union -- of betraying their promise from the end of the Cold War of not expanding eastwards.
"Russia has a peaceful foreign policy, but has the right to defend its security," Putin said a day after the Biden call.
He added that letting NATO approach Russia's borders without reacting would amount to "criminal inaction".
The Kremlin has also spent weeks denouncing US-led military exercises in the Black Sea.
At the same time, Moscow wants to reconnect with Washington on key issues like strategic stability and Iran.
Putin said his talk with Biden was "constructive."
What about the US?
After and during his call with Putin, Biden threatened his Russian counterpart with sanctions "like he's never seen before" in the event of an attack on Ukraine.
Washington has also said it is ready to beef up its military presence in eastern Europe.
But the US leader ruled out sending troops in support of Ukraine.
He is not obliged to because Kiev is not a member of NATO, an alliance whose members pledge assistance in the event of aggression.
The US and European countries have made clear on numerous occasions that Ukraine's membership to NATO is not on the cards -- much to Kiev's annoyance.
Washington also wants to resume cooperation with Russia on issues beyond Ukraine, in particular disarmament and cybersecurity.
What is the situation on the ground?
The situation on the frontline of the conflict between Kiev and pro-Russia rebels is tense, but does not seem to have deteriorated significantly in recent weeks.
The OSCE said Thursday that while the number of ceasefire violations in the last two weeks was "lower" compared to previous weeks, the situation on the frontline was "still of concern".
On the Russian side, tens of thousands of troops are stationed near the border with Ukraine. But a large part of them have been there since Spring.
Throughout the autumn, Kiev even played down US claims of an imminent Russian invasion, before backing Washington's accusations.