The Pentagon ordered 8,500 troops on higher alert to potentially deploy to Europe as part of a NATO “response force” amid growing concern that Russia could soon make a military move on Ukraine. President Joe Biden consulted with key European leaders, underscoring U.S. solidarity with allies there.

Putting the U.S.-based troops on heightened alert for Europe on Monday suggested diminishing hope that Russian President Vladimir Putin will back away from what Biden himself has said looks like a threat to invade neighboring Ukraine.

At stake, beyond the future of Ukraine, is the credibility of a NATO alliance that is central to U.S. defense strategy but that Putin views as a Cold War relic and a threat to Russian security. For Biden, the crisis represents a major test of his ability to forge a united allied stance against Putin.

US president Joe Biden has insisted there was “total” unity among western powers after crisis talks with European leaders on how to deter Russia from an attack against Ukraine, as Downing Street warned of “unprecedented sanctions” against Moscow should an invasion take place. .

“I had a very, very, very good meeting – total unanimity with all the European leaders,” Biden told reporters shortly after finishing a one hour and 20 minute video conference on Monday with allied leaders from Europe and Nato.

The US has put 8,500 troops on high alert to deploy to Europe as Nato reinforced its eastern borders amid growing tensions over Ukraine.

In London, prime minister Boris Johnson’s office also said “the leaders agreed on the importance of international unity in the face of growing Russian hostility.”

Downing Street said the group stressed that diplomatic discussions with Russia remain the first priority, but said the nation would be hit with “swift retributive responses” if a “further Russian incursion into Ukraine” takes place.

“The leaders agreed that, should a further Russian incursion into Ukraine happen, allies must enact swift retributive responses including an unprecedented package of sanctions,” Downing Street said following the discussions lasting over an hour.

Washington is trying to maintain transatlantic and Nato unity against Russia, which supplies about 40% of the European Union’s natural gas.

The meeting came days after Biden revealed behind-the-scenes divisions among the Nato allies on how severe the response would be and as Germany faces criticism from Kyiv over its refusal to send defensive weapons to Ukraine.