"Neptune strike 22 is going to run through February 4 and it's designed to demonstrate NATO's ability to integrate the high-end maritime strike capabilities of an aircraft carrier strike group to support the deterrence and defense of the Alliance," said Pentagon spokesman John Kirby.

Kirby said preparations for the exercise had begun in 2020 and that it had nothing to do with fears that Russia could invade Ukraine, on whose borders Moscow has massed tens of thousands of troops.

"Neptune Strike 2022" however does not appear on the list of scheduled exercises for 2022 published by NATO on its website on December 14.

"The exercise itself is not designed against the kinds of scenarios that that might happen with respect to Ukraine," Kirby said.

"It really is a NATO maritime exercise to test... a wide range of maritime capabilities that we want to make sure we continue to improve."

The announcement came a day after Russia announced its own all-out naval exercises, with more than 140 warships and about 10,000 soldiers taking part in January and February in maneuvers that will take place in the Atlantic, the Arctic, the Pacific and the Mediterranean.

Kirby acknowledged that tensions with Russia had sparked discussions among the allies on the holding of exercises.

"There was due consideration about -- given tensions right now -- about our exercise posture. And after all that consideration and discussion with our NATO allies, the decision was made to move ahead," he told reporters.

The USS Harry Truman and its carrier group have been in the Mediterranean since mid-December. The aircraft carrier was to join the area of operations of the central command (Centcom) but US Defense Secretary Lloyd Austin decided at the end of December to keep it in the Mediterranean to "reassure" Europeans amid tensions with Russia.

Without mentioning the friction around Ukraine, where Kyiv and its western allies accuse Moscow of having massed 100,000 troops at the country's borders in anticipation of a possible invasion, the Pentagon had stressed the change of course "reflected the need for a continued presence in Europe."