A Russian flag flies outside a Ukrainian military base as Armed men believed to be Russian military patrol outside on 12 March 2014 in Simferopol, Ukraine Russian troops have seized key sites in Crimea ahead of Sunday's vote. Photo: BBC NEWS

The US has threatened action if Russia does not remove its troops from Crimea.

Germany's Angela Merkel has said Moscow faces "massive" political and economic damage if it refuses to change course.

Russia was exploiting the weakness of neighbouring Ukraine, rather than acting as a partner for stability, the chancellor said on Thursday, adding that there was no military solution to the crisis.

The Russian military and pro-Russian armed men moved in to seize key sites in Crimea - an autonomous region of Ukraine whose population is mainly ethnic Russian - in late February after the fall of President Viktor Yanukovych.

'Serious measures'

The German chancellor has threatened an escalating series of EU measures if Russia does not relax the tension in Ukraine.

In a statement to the Bundestag, she said political and diplomatic measures, rather than military action, were the way to resolve the crisis.

"If Russia continues on its course of the past weeks, it will not only be a catastrophe for Ukraine," she said.

"We would not only see it, also as neighbours of Russia, as a threat," Mrs Merkel told MPs. "And it would not only change the European Union's relationship with Russia. No, this would also cause massive damage to Russia, economically and politically."

Although she did not use the word sanctions, she warned that if there was no progress in the next few days, measures with "serious economic impact" would be taken.

"The territorial integrity of Ukraine cannot be called into question," she said.

EU foreign ministers are due to meet on Monday to consider their next steps. EU leaders have already suspended talks with Russia on easing visa restrictions as well as preparations for a G8 meeting in Sochi in June.

The leaders of the G7 group of industrial nations - Britain, Canada, France, Germany, Italy, Japan and the US - and the EU on Wednesday threatened to "take further action, individually and collectively" if Russia continues on its course.

Meanwhile, the head of the Duma (Russian parliament) international relations committee has acknowledged that Russian troops are involved in controlling Crimea - a break from Moscow's official position that the armed, uniformed men are "self-defence units".

"There are some military units there, which are in position in case there is an aggression, expansion from Kiev," Leonid Slutsky told Ekho Moskvy radio.

It was not a "large-scale military operation" but they were there to protect people and ensure there was no bloodshed, he said.

Welcoming Ukraine's interim prime minister Arseniy Yatsenyuk to the White House earlier, US President Barack Obama pledged to "stand with Ukraine" in its dispute.