If President Vladimir Putin "moved in, there would be an increasing request from eastern flank allies and a positive response from the United States for additional forces and capabilities and exercises," the official said.

Moscow seized Crimea from Ukraine in 2014 and has since backed separatists fighting Kiev in the east of the country. The conflict has left more than 13,000 dead.

Washington and Kiev say Moscow has massed troops near Ukraine's borders and accuse Russia of planning an invasion.

Biden said Friday he would make it "very, very difficult" for Russia to do so.

Moscow has denied any bellicose intentions and accused the West of provocation, particularly with military exercises in the Black Sea, which it sees as part of its sphere of influence.

Amid the rising tensions, the two leaders are due to hold a video call on Tuesday.

A key question hanging over the talks is whether Putin might actually start a cross-border offensive, or whether he is using the troops to pressure Biden for guarantees ex-Soviet Ukraine will never become a NATO launchpad.

The senior US official said that Biden "will be speaking later today with key European allies to coordinate his message" ahead of the call.

The official added that Secretary of State Antony Blinken would speak with Ukraine's President Volodymyr Zelensky ahead of the secure video link -- and that Biden would brief Zelensky after the talks.

The Kremlin said earlier Monday that Moscow is not expecting "breakthroughs" from the call.

Biden and Putin have a daunting list of other differences to air, from Russia's harsh treatment of dissidents to the presence of ransomware hackers on Russian soil to Moscow's support for the repressive regime in Syria.