By Stefan J. Bos
Biden's comments came shortly before Britain's defense secretary visited Moscow on Friday amid concerns about massive Russian war games near Ukraine.
The president suggested that Russian troops may soon enter the country after annexing Ukraine's Crimea peninsula in 2014. "It's not like we're dealing with a terrorist organization are dealing with one of the largest armies in the world, and things could get crazy quickly," Biden stressed.
Close to 130,000 Russian forces are believed to be already near Ukraine's borders. Russia has also been accused of actively supporting pro-Russian separatists in eastern Ukraine, charges Moscow denies.
Yet, Biden warned he would not send troops to rescue Americans refusing to leave if Moscow invades Ukraine. "That's a world war when Americans and Russians start shooting at one another; we're in a very different world than we've ever been in," Biden said. And he wondered: "How do you do that? How do you even find them (Americans)? This is not like..."
However, "I am hoping that if in fact, he [Russian President Vladimir Putin]is foolish enough to go in, he is smart enough not to in fact do anything that would negatively impact on American citizens."
But Biden added: "Look what I have asked is: American citizens should leave, should leave now."
His comments released by NBC News television came while Britain's defense secretary Ben Wallace visited Moscow on Friday to discuss ending the East-West standoff.
Wallace arrived amid massive Russian war games near Ukraine, including Belarus. Moscow also sends warships for upcoming drills into the Black and Azov seas.
NATO has responded by sending troops, including several thousand additional U.S. forces, into eastern Europe. And the United States Navy says it has deployed several destroyers into European waters.
It was not immediately clear how Wallace could ease tensions.His visit came a day after British Foreign Secretary Liz Truss warned Russia in Moscow that attacking its neighbor would "have massive consequences and carry severe costs."
Russia says it has no intention to invade Ukraine. But it wants the West to keep Ukraine and other former Soviet countries out of the NATO military alliance.
Moscow also says NATO should refrain from deploying weapons in Eastern Europe. But the West has refused to accept these demands.