Pro-Russian protesters stormed Ukraine's naval headquarters in Crimea today after Moscow claimed the peninsula and the first casualties ratcheted up the stakes in the worst East-West crisis since the Cold War.

A Ukrainian navy spokesman said the crowd of a few hundred irate activists in Ukraine's port city of Sevastopol had forced a group of officers to barricade themselves inside the building to avoid a direct confrontation.

"There are about 200 of them, some wearing balaclavas. They are unarmed and no shots have been fired from our side," spokesman Sergiy Bogdanov told AFP.

"The officers have barricaded themselves inside the building," he said, adding that the officers had no intention of using their weapons.

A defiant President Vladimir Putin had brushed aside global indignation and Western sanctions yesterday to sign a treaty absorbing the flashpoint Ukrainian peninsula and expanding Russia's borders for the first time since World War II.

The historic and hugely controversial moment came less than a month after the ouster in Kiev of a Moscow-backed regime by leaders who spearheaded three months of deadly protests aimed at pulling Ukraine out of the Kremlin's orbit for the first time.

Putin responded by winning the right to use force against his ex-Soviet neighbour and then using the help of local militias to seize the Black Sea region of Crimea -- the warm water outlet for Russian navies since the 18th century.

The explosive security crisis on the EU's eastern frontier now threatens to reopen a diplomatic and ideological chasm between Russia and Western powers not seen since the tension-fraught decades preceding the 1989 fall of the Berlin Wall.

"Russia's political and economic isolation will only increase if it continues down this path and it will in fact see additional sanctions by the United States and the EU," US Vice President Joe Biden warned yesterday while paying a visit to Poland aimed at reassuring former Soviet satellites of Washington's backing in the face of the Kremlin's expansionist threat.