US ambassador defiantly refuses to leave Russia: report
Vladimir Putin (Shutterstock)

On Monday, Axios reported that John Sullivan, the U.S. ambassador to Russia, is refusing to leave the country — despite being "advised" by Kremlin officials to get out.

"Russia's foreign ministry announced Friday it would expel 10 American diplomats and bar current officials, such as Attorney General Merrick Garland, from visiting Russia," reported Jonathan Swan. "But the Russians didn't expel Sullivan. Instead, the Kremlin summoned him to meet with a top foreign policy official, Yuri Ushakov, who recommended he go back to Washington for consultations with Biden officials."

"Sullivan's view, according to people familiar with his thinking, is that if Putin wants him to leave, he'll have to force him," continued the report.

Russia imposed the expulsions in response to new U.S. sanctions on a number of the country's political and intelligence officials — a belated retaliation for the Solar Winds hack and military aggression in the Ukraine.

Also on Monday, US Defense Department spokesman John Kirby warned about the size of Russia's troop build-up on the border with Ukraine.

"It is the largest buildup we've seen certainly since 2014, which resulted in the violation of Ukrainian sovereignty and territorial integrity," Kirby told a news conference. "It is certainly bigger than the last one in 2014."