US President Barack Obama on Monday vowed Western unity in punishing Moscow for annexing Crimea, ahead of crisis talks that could see Russia excluded from the G8 club of rich nations.

US President Barack Obama on Monday vowed Western unity in punishing Moscow for annexing Crimea, ahead of crisis talks that could see Russia excluded from the G8 club of rich nations.

In Ukraine itself, the country's acting president announced that its troops had been given orders to withdraw from Crimea after the fall of another military base to Kremlin troops.

"Europe and America are united in our support of the Ukrainian government and the Ukrainian people, we're united in imposing a cost on Russia for its actions so far," Obama told journalists at Amsterdam's Rijksmuseum.

Obama then headed to The Hague where he has called an emergency Group of Seven summit to discuss what steps to take in the worst East-West standoff since the Cold War.

"The national security and defense council has reached a decision, under instructions from the defense ministry, to conduct a redeployment of military units stationed in the Autonomous Republic of Crimea," acting Ukraine president Oleksandr Turchynov told lawmakers in the face of the latest seizure.