Ukraine warned Sunday it was on the brink of disaster and called up all military reservists after Russia's threat to invade its neighbour drew sharp rebukes from the United States and its Western allies.
US Secretary of State John Kerry upped the stakes for Russian President Vladimir Putin by bluntly warning that Moscow risked losing its coveted place among the Group of Eight nations over its deployment of troops in Crimea.
The dramatic escalation in what threatens to blow up into the worst crisis between Moscow and the West since the Cold War came as pro-Kremlin forces seized key government buildings in Crimea and were blockading military bases on the strategic Black Sea peninsula.
World leaders huddled for urgent consultations across global capitals after Russia's parliament voted Saturday to allow Putin to send troops into its western neighbour -- a decision US President Barack Obama branded a "violation of Ukrainian sovereignty".
The former Soviet nation's new pro-Western Prime Minister Arseniy Yatsenyuk also warned any invasion "would mean war and the end of all relations between the two countries."
"We are on the brink of a disaster," Yatsenyuk told the nation of 46 million in a televised address. "This is not a threat. This is a declaration of war on my country."
Pro-Moscow gunmen intensified their grip Sunday on large swathes of the rugged flashpoint peninsula that has housed Russian navies since the 18th century.
Witnesses said Russian soldiers had blocked about 400 Ukrainian marines at a base in the eastern port city of Feodosiya and were calling on them to surrender and give up their arms.