United States Secretary of State Antony Blinken has said the "imminent" threat of Russian military action in Ukraine justifies evacuating the US embassy in Kyiv.

He said this after Ukraine's President urged calm, saying the biggest enemy was panic.

More than a dozen countries have urged their citizens to leave Ukraine.

Moscow, with more than 100,000 troops near the border, has denied it plans to invade.

Meanwhile, in a phone call, President Joe Biden warned Russian leader Vladimir Putin of swift and severe costs if Russia sends in troops.

British Defence Secretary Ben Wallace, meanwhile, compared recent Western diplomatic efforts to stop an invasion to the appeasement of Nazi Germany.