The US has put up to 8,500 troops on alert to be ready to deploy to Europe amid fears Russia may be preparing to invade Ukraine.
The Pentagon announced the move on Monday, saying the troops could be deployed, potentially at very short notice, should the Nato alliance activate a rapid response force.
Pentagon spokesman John Kirby stressed that no decision had yet been made on whether to deploy the troops.
“What this is about is reassurance to our Nato allies,” Mr Kirby said.
Russia has massed an estimated 100,000 troops near Ukraine’s border, demanding that Nato promise it will never allow Ukraine to join and that other actions, such as stationing alliance troops in former Soviet bloc countries, be curtailed. However, Moscow denies it is planning an invasion.
Earlier on Monday, Nato said it is bolstering its “deterrence” in the Baltic Sea region.
Denmark is sending a frigate and deploying F-16 warplanes to Lithuania; Spain is sending four fighter jets to Bulgaria and three ships to the Black Sea to join Nato naval forces; and France stands ready to send troops to Romania. The Netherlands also plans to send two F-35 fighter aircraft to Bulgaria from April.
The alliance will “take all necessary measures to protect and defend all allies,” Secretary-General Jens Stoltenberg said.
“We will always respond to any deterioration of our security environment, including through strengthening our collective defence.”
In Washington, Pentagon leaders proposed a range of options for President Joe Biden to reinforce the US military presence in Eastern Europe and the Baltics as a demonstration of American commitment, according to two officials who spoke on condition of anonymity to discuss internal planning.
Mr Biden was set to hold a video call with several European leaders on the Russian military buildup and potential responses to an invasion, the White House said.
In Moscow, Kremlin spokesman Dmitry Peskov said it was Nato and the US who were behind the escalating tensions, not Russia.
“All this is happening not because of what we, Russia, are doing. This is happening because of what NATO, the US are doing,” Mr Peskov told reporters.
The Nato announcement came as European Union foreign ministers sought to put on a fresh display of unity in support of Ukraine, and paper over concerns about divisions on the best way to confront any Russian aggression.
In a statement, the ministers said the EU has stepped up sanction preparations and they warned that 2any further military aggression by Russia against Ukraine will have massive consequences and severe costs”.
Separately, the EU also committed to increase financial support for embattled Ukraine, vowing to push through a special package of 1.2 billion euros (£1 billion) in loans and grants as soon as possible.
The UK earlier said it is withdrawing some diplomats and dependents from its Kyiv Embassy.
Prime Minister Boris Johnson said an invasion was not inevitable, but “the intelligence is pretty gloomy.” He added that “I think that sense can still prevail.”
