Obama vows Western unity ahead of Ukraine crisis summit

We're united in imposing a cost on Russia for its actions so far, says Obama

US President Barack Obama today 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.

Russian troops have rapidly overrun the flashpoint Black Sea peninsula since the fall of a pro-Moscow government in Kiev a month ago.

"The national security and defence council has reached a decision, under instructions from the defence 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.

A senior official in the pro-Russian regional government later said that all troops loyal to Ukraine had left their bases in Crimea.

A top NATO commander had warned yesterday that the Western military alliance was carefully watching massive Russian troop formations on the eastern border of Ukraine that could theoretically make a push across the vast ex-Soviet country at any point.

Paratroopers and armoured personnel carriers stormed the naval base in Feodosia in eastern Crimea in the early hours today, with vehicles seen leaving the base carrying Ukrainian marines whose hands had been tied.

Russia's takeover of Crimea, which it views as a reunification, has forced Western leaders to rethink their relationship with Moscow after a post-Cold War period in which they sought to usher Russia into the broader international community.