The alert order issued by the defence secretary, Lloyd Austin, reduces the number of days it would take to deploy but it is not itself an order to deploy.

The USS Harry S Truman aircraft carrier, along with its strike group and air wing, joined patrolling activities across the Mediterranean Sea on Monday, the first time since the cold war that a full US carrier group has come under Nato command.

Kirby said: “In the event of Nato’s activation of the NRF or a deteriorating security environment, the United States would be in a position to rapidly deploy additional brigade combat teams, logistics, medical, aviation, intelligence, surveillance and reconnaissance, transportation and additional capabilities into Europe.”

Any deployment in Europe, he said, “is really about reassuring the eastern flank of Nato” of the US readiness to come to the defence of alliance members. The force would not be deployed in Ukraine, which is not a Nato member. There are currently about 150 US military advisers in the country, and Kirby said there were no plans at present to withdraw them.

Jen Psaki, the White House spokesperson, said the US had “a sacred obligation to support the security of our eastern flank countries”.

“We are talking to them about what their needs are and what security concerns they have. So I wouldn’t say it’s a response to an abrupt moment. It’s a part of an ongoing contingency planning process and discussion,” Psaki said.

Earlier on Monday, Jens Stoltenberg, Nato’s secretary general, said the “deteriorating security situation” had driven the military alliance to bolster its “collective defence”.