The US Department of Defence on Monday said it was putting up to 8,500 US-based troops on "heightened alert" to join a Nato response force that could be called up in the crisis at the Ukrainian border with Russia, where Moscow has amassed about 127,000 troops in preparation for a possible invasion.
Pentagon spokesman John Kirby emphasised that no final decision has been made on whether to deploy the US forces. The heightened alert status means troops will be ready to deploy by the end of the week, if called upon.
This "will ensure that the United States and our commitment to the NRF [Nato Response Force] is consistent with their readiness for rapid deployment ... if activated," Mr Kirby said.
The NRF force comprises 40,000 Nato troops, the Pentagon said.
While Mr Kirby did not specify the countries where these troops could deploy, he suggested the eastern flank nations bordering Russia - and not Ukraine itself.
"This is really about reassuring the eastern flank of Nato ... [Defence Secretary Lloyd Austin] wants us postured to be ready for any other contingencies as well," he said.
Russia has criticised Nato deployments to these countries and demanded a lighter Nato footprint around its borders.
Mr Kirby said there is currently "no intent by the Russians right now to de-escalate".
The heightened alert status "sends a clear signal to Mr Putin that we take our responsibility seriously."
"It is about making sure that [the US troops] had plenty of time to prepare. And that's all we're doing at this point," he added.
Mr Kirby said the troops have so far not been assigned a mission. The US forces include "additional brigade combat teams, logistics personnel, medical support, aviation support, intelligence, surveillance and reconnaissance".
The White House on Monday said discussions around these deployments have been ongoing.
"We’ve always said we would reinforce our allies on the eastern flank, and those conversations and discussions have certainly been part of what our national security officials have been discussing with their counterparts now for several weeks," White House Press Secretary Jen Psaki said.
President Joe Biden is scheduled to speak with European leaders late on Monday.