KIEV  - 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 a sharp rebuke from the United States and NATO.
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 control of key government buildings and airports in the predominantly Russian-speaking Crimean peninsula.
Russia's parliament voted Saturday to allow President Vladimir Putin to send troops into its western neighbour - a decision US President Barack Obama swiftly branded a "violation of Ukrainian sovereignty".
And NATO's chief declared that Russia's actions in the former Soviet state were a threat to peace and security in Europe.
Ukraine'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 in a televised address. "This is not a threat. This is a declaration of war on my country."
As world leaders held urgent meetings on the crisis, pro-Moscow gunmen were controlling large swathes of the rugged Black Sea peninsula that has housed Kremlin navies since the 18th century.
Witnesses said Russian soldiers had also blocked about 400 Ukrainian marines at their base in the eastern Crimean port city of Feodosiya and were calling on them to surrender and give up their weapons.
The largely untested interim team that took power in Kiev just a week ago braced for Moscow's first possible invasion of a neighbour since a brief 2008 confrontation with Georgia by putting its military on full combat alert on Saturday.
Ukraine's navy chief announced he had switched allegiance to the pro-Russian authorities of the flashpoint peninsula of Crimea, a day after he was appointed to the post by interim leader Oleksandr Turchynov.
"I swear to execute the orders of the (pro-Russia) commander-in-chief of the Autonomous Republic of Crimea," Denis Berezovsky said in a televised statement from inside the Crimean headquarters of the Russian Black Sea Fleet, adding that he "swears allegiance to the residents of the Autonomous Republic of Crimea".
Ukraine's national security and defence council said it would call up all reservists and start preparations for a possible invasion from its giant neighbour to the east.
Ukraine says Russia has already sent 30 armoured personnel carriers and 6,000 additional troops into Crimea to help pro-Kremlin militia gain broader independence from Kiev.
Putin said Saturday he had a duty to protect ethnic Russians in Crimea and southeastern swathes of Ukraine which have ancient ties to Moscow and look on Kiev's new pro-EU leaders with disdain.
But NATO chief Anders Fogh Rasmussen, speaking before an emergency meeting of the transatlantic alliance, told Russia to stop its military activity and threats against Ukraine, saying its action threatened "peace and security in Europe". The United States and its Western allies have threatened to boycott the June G8 summit in the Russian Black Sea resort of Sochi. The city was the host of last month's Winter Olympics, a $51 billion extravaganza which along with the football World Cup in 2018 are meant to highlight Russia's return to prosperity and global influence under Putin's rule.
The Russian parliament's unanimous vote to authorise the use of force triggered an international outcry whose ferocity underscored the growing distance between Moscow and the West during Putin's 14 years in power.
The vote came after a three-month crisis in the culturally splintered nation of 46 million - long fought over by Moscow and the West - culminated in a week of carnage last month that claimed nearly 100 lives and led to the ouster of pro-Kremlin president Viktor Yanukovych.
The Kremlin appeared stunned by the loss of its ally and Kiev's subsequent vow to seek EU membership - a decision that would shatter Putin's dream of reassembling a powerful economic and military post-Soviet bloc. The parliament vote drew what one US official described as a "candid and direct" response from Obama.
The White House described a charged 90-minute call in which Obama told Putin that Russia's reported deployment of troops outside bases Moscow leases from Kiev in Crimea had already broken international law.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said. It said Obama told Putin his actions were a "breach of international law, including Russia's obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine."
US Secretary of State John Kerry also hosted a joint call with six foreign ministers from Europe and Canada as well as EU foreign policy chief Catherine Ashton and the Japanese envoy to Washington "to coordinate on next steps".
The Kremlin's account of Putin's conversation with Obama was equally blunt. It said Putin drew the US leader's "attention to the provocations and crimes of ultranationalist elements, which are effectively being encouraged by the current authorities in Kiev".
"In case of the further spread of violence in the eastern regions of Ukraine and Crimea, Russia reserves the right to protect its interests and those of the Russian-speaking population," Putin said.
His actions have received overwhelming support from senior lawmakers and state-controlled media in Moscow.
They are portraying the crisis as a battle between dangerous ultra-nationalists and Russian-speakers who are coming under increasing attack.
"The situation in Ukraine is consolidating Russia's entire civil society," powerful lower house of parliament lawmaker Leonid Slutsky told reporters.
Russia's three main news agencies also issued identical reports from Crimea Sunday claiming that Ukrainian troops were switching allegiance to the local pro-Kremlin authorities "en masse".
In Kiev however, about 50,000 people massed on Independence Square - the crucible of both the latest wave of protests and the 2004 Orange Revolution that first nudged Kiev on a westward path - in protest at Russia's sabre rattling.
"We will not surrender," the huge crowd chanted under grey skies.
Ukraine's prime minister had assured the nation Saturday he was "convinced" Russia would not launch an offensive because Moscow realised it would put an end to relations between two neighbours with centuries of shared history. Kremlin spokesman Dmitry Peskov also said Saturday that "for the moment, this decision (to invade Ukraine) has not been taken".