Ukrainian and Western officials are worried that a Russian military build up near their country could signal plans to invade it.
The Kremlin accused the Western backers of making the claims to cover up their own aggressive designs.
It is not clear if the Russian troop concentration heralds an attack. Russian President Vladimir Putin has pushed for Western guarantees before NATO's expansion to Ukraine, which could mean an attempt to back up his message.
The current tensions are shown here.
What are the root causes of the Russia-UKRAINE dispute?
As the USSR broke up in 1991, Ukraine won independence, which was part of the Russian empire for centuries. The country has moved to forge closer ties with the West.
mass protests led to the ousting of Ukrainian President Viktor Yanukovych after he rejected an association agreement with the European Union in favor of closer ties with Moscow. Russia threw its weight behind the rebellion in the east of the country.
Russia was accused of sending troops and weapons to support the rebels. Moscow said that Russians who joined the rebels were volunteers.
More than 14,700 people have died in the fighting in the eastern part of the country.
Efforts to reach a political settlement have failed, and skirmishes have continued along the tense line of contact despite the peace agreement brokered by France and Germany.
The war fears were fueled by a spike in cease-fire violations in the east and a Russian troop concentration near the Ukrainian border, but the tensions subsided after Moscow pulled back the bulk of its forces.
The Russian military build up is still going on.
According to the U.S. intelligence officials, Russia is planning to deploy an estimated 175,000 troops and almost half of them are already stationed along various points near the Ukrainian border in preparation for a possible invasion that could begin as early as early 2022.
Russia has kept many of its troops close to the border with Ukraine following war games in the fall.
The Ukrainian Defense Ministry said that units of the Russian 41st army are in Yelnya, a town about 160 miles north of the Ukrainian border.
The number of Russian troops in the area is estimated at 94,300, according to the Ukrainian Defense Minister.
The commander-in-chief of the Ukrainian armed forces says that Russia has about 2,100 military personnel in the east of the country and that Russian officers hold all commanding positions. Moscow has denied that it has troops in eastern Ukraine.
Russia has not provided any information about its troop numbers or locations, saying that their deployment on its own territory shouldn't worry anyone.
What do Moscow want?
The West was criticized by the Kremlin for failing to encourage Ukrainian compliance with the peace deal. The agreement required Ukraine to grant broad autonomy to the rebel regions and offer a sweeping amnesty to the rebels.
The Ukrainian government has pointed to cease-fire violations by Russia-backed rebels and insists there is a continuing Russian troop presence in the east.
Russia has rejected a four-way meeting with France, Germany, and Ukraine because of Ukraine's refusal to abide by the 2015 agreement.
Russia criticized the U.S. and its NATO allies for giving weapons to the Ukrainians and for holding drills that encourage them to regain the rebel-held areas.
Putin said a military attempt by Ukraine to regain the east would have grave consequences for the country.
Russian and Ukrainians are described as one people by the Russian president.
Putin has made it clear that NATO membership for Ukraine is a red line for Moscow, and that some NATO members are planning to set up military training centers in the country. He said that would give them a military foothold.
Putin said last week that Russia would seek long-term security guarantees from the U.S. and its allies that would exclude any further NATO moves eastward and the deployment of weapons systems that threaten us in close vicinity to Russian territory.
He said that NATO placing its military infrastructure closer to Russia and the West offering to engage in substantive talks on the issue made him believe that Moscow would need legal guarantees.
Many former US and NATO diplomats say any Russian demand to Biden would be a non-starter. Biden said he doesn't accept anyone's red line.
Is the threat of a Russian invasion real?
Russia rejected the idea of an invasion plot as a Western ploy to distract attention from a Ukrainian plan to attack the east. Ukraine denies such plans.
Some observers think that the troop build up is a demonstration by Putin that Russia is prepared to raise the stakes to convince NATO to respect its red lines and stop sending troops and weapons to Ukraine.
Putin was happy that Moscow's warnings finally have some traction and caused a stress in the West. It is necessary to keep them in that condition for as long as possible so that they don't stage a conflict on our western borders that we don't need.
U.S. officials pointed to Russia's past behavior as a cause for concern, despite the fact that Moscow's intentions are unclear.
Biden said Friday that a set of new initiatives coming from his administration are intended to deter Russian aggression.
Dasha Litvinova is from Moscow.