Ukraine warned Sunday it was on the brink of disaster and called up all military reservists after Russia's threat to invade its neighbor drew sharp rebukes from the United States and its Western allies.

U.S. 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.

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World leaders huddled for urgent consultations across global capitals after Russia's parliament voted Saturday to allow Putin to send troops into its western neighbor -- a decision 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.

Ukraine's defense ministry said 1,000 armed fighters and 20 trucks were also blocking the entrance to a border guard unit in the southern Crimean town of Perevalne.

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Fearing Moscow's first possible invasion of a neighbor since a brief 2008 confrontation with Georgia, the largely untested interim team that took power in Kiev just a week ago put its military on full combat alert on Saturday and on Sunday announced the call-up of all reservists.

Ukraine says Russia has already sent 30 armored personnel carriers and 6,000 additional troops into Crimea to help pro-Kremlin militia gain broader independence from Kiev.

Putin said Saturday it was his duty to protect ethnic Russians in Crimea and southeastern swathes of Ukraine that have ancient ties to Moscow and look on Kiev's new pro-EU leaders with disdain.

Russian officials also argued they had no need to turn for permission from the UN Security Council -- as Putin had demanded for any Western action in Syria -- because the well-being of their own citizens was at stake.

NATO chief Anders Fogh Rasmussen, holding urgent talks in Brussels, told Russia to put an immediate end to its military activities, saying it "threatens peace and security in Europe".

German Frank-Walter Steinmeier spoke ominously of preventing "a new division of Europe" while France and Britain called for negotiations to be organised between Moscow and Kiev, either directly or through the United Nations.

In the most immediate response to Russia's actions in the country on the eastern edge of Europe, the U.S. and its Western allies pulled out of preparatory meetings this week for the June G8 summit in the Russian Black Sea resort of Sochi.

Kerry went one step further by warning Putin that "he is not going have a Sochi G8, he may not even remain in the G8 if this continues.

"He may find himself with asset freezes, on Russian business, American business may pull back, there may be a further tumble of the ruble."

Russia was admitted to the G8 in 1998 in recognition of the late president Boris Yeltsin's democratic reforms -- a spot the Kremlin has coveted as a recognition of its post-Soviet might.