NATO said on Monday it was putting troops on standby and reinforcing eastern Europe with more ships and fighter jets, in what Russia denounced as Western hysteria in response to the build-up of troops on the Ukraine border.
The U.S. Department of Defense said about 8,500 American troops were on heightened alert and were awaiting orders to deploy to the region if Russia invades Ukraine.
Tensions are high after Russia massed 100,000 troops in the vicinity of its neighbour's border, surrounding Ukraine with forces from the north, east and south.
Russia denies planning an invasion and Moscow is citing the Western response as evidence that Russia is the target, not the instigator of aggression.
President Joe Biden, pushing for transatlantic unity, held an 80-minute secure video call with a number of European leaders on Monday from the White House Situation Room to discuss the Ukraine crisis.
Biden told reporters that they had a very good meeting with the Europeans, including the leaders of Germany, France, Italy, Britain and Poland. He said there was total unanimity. A White House statement said the leaders discussed their joint efforts to deter Russian aggression against Ukraine, including preparations to impose massive consequences and severe economic costs on Russia for such actions, as well as strengthen security on NATO's eastern flank. Secretary General Jens Stoltenberg earlier said NATO would take all necessary measures after being welcomed by a series of deployments announced by alliance members in recent days. Stoltenberg said that we will always respond to any deterioration of our security environment, including strengthening our collective defence.
He told a news conference that the enhanced presence on NATO's eastern flank could also include the deployment of battlegroups in the southeast of the alliance.
NATO has so far 4,000 troops in multinational battalions in Estonia, Lithuania, Latvia and Poland, backed by tanks, air defences and intelligence and surveillance units.
U.S. officials said the Pentagon was completing its efforts to identify specific units that it could deploy to NATO's eastern flank.
One official said up to 5,000 troops could be deployed, while a NATO diplomat said Washington was considering moving some troops from western Europe to eastern Europe in the coming weeks.
Denmark, Spain, France and the Netherlands were all planning to send troops, planes or ships to eastern Europe, NATO said. Ukraine shares borders with four NATO countries: Poland, Slovakia, Hungary and Romania.
A Polish official said Warsaw would draw the line at sending troops to Ukraine.
Britain withdraws staff and dependents from its embassy in Ukraine as a result of the growing tensions, a day after the United States said it was ordering diplomats' family members to leave. U.S. diplomats are allowed to leave freely.
Dmitry Peskov, a Kremlin spokesman, accused the West of hysteria and accused it of lying. We see statements from the North Atlantic Alliance about reinforcement, pulling forces and resources to the eastern flank. The tensions are growing, he said.
This is not happening because Russia is doing what we are doing. This is happening because of what NATO and the U.S. are doing and the information they are spreading. The possibility of a Russian attack quashed demand for riskier assets such as bitcoins and boosted the dollar and oil, as global stock markets skidded. Russian stocks and bonds fell as the rouble hit a 14 month low against the dollar.
Russia has used its troop build-up to draw the West into discussions after presenting demands to redraw Europe's security map. It wants NATO to never admit Ukraine and to pull back troops and weapons from former Communist countries in eastern Europe that joined it after the Cold War.
Washington says that demands are non-starters but it is ready to discuss other ideas on arms control, missile deployments and confidence-building measures.
Russia is waiting for a written U.S. response this week after talks last Friday -- the fourth round this month -- haven't produced a breakthrough.
Asked whether he thought an invasion was imminent, British Prime Minister Boris Johnson told broadcasters that intelligence was pretty gloomy on this point but that sense can still prevail. He warned that invading Ukraine would be a painful, violent and bloody business for Russia.
The United States and the European Union, wary of Russia's intentions since it seized Crimea and backed separatists fighting government forces in eastern Ukraine in 2014, have told Russia it will face crippling penalties if it attacks again.
EU foreign ministers meeting in Brussels warned Russia it would face massive consequences, but they are divided over how tough it is to be on Moscow and did not say what the consequences might be.
Ukrainian President Volodymyr Zelenskiy told EU President Charles Michel, who was also on the call with Biden, that it was important for Kyiv that the EU showed unity.
The Ukraine will not fall for provocations and will remain calm and restrained, along with its partners, he said.
The EU executive body, the European Commission, proposed a 1.2 billion euro $1.36 billion financial aid package to help Ukraine mitigate the effects of the conflict.
According to a Russian delegation, political advisers from Russia, Ukraine, France and Germany would meet in Paris on Wednesday for talks about resolving the conflict in eastern Ukraine, in which more than 15,000 people have been killed since 2014. Previous efforts didn't yield a breakthrough.