© UnknownNATO General Secretary: Anders Fogh Rasmussen
Ukraine has mobilised for war amid warnings from Nato that Russia's annexation of Crimea "threatens peace and security in Europe".

With tension nearing boiling point, Anders Fogh Rasmussen, the Nato Secretary General, vowed the organisation would stand by Ukraine, a nation of 46 million which occupies a vital strategic position between Europe and Russia.

Speaking before he chaired an emergency meeting of ambassadors from the 28 Nato member states, he said: "Russia must stop its military activities and its threats."

The United States dramatically instensifed pressure on Moscow, threatening to remove Russia from the G8 club of developed economies.

John Kerry, the US Secretary of State, condemned Russia for what he called an "incredible act of aggression" and threatened "very serious repercussions", including Russia's possible expulsion from the G8.

"You don't just, in the 21st century, behave in 19th century fashion by invading another country on a completely trumped-up pretext," he said.

Mr Kerry announced a surprise trip to Kiev this week in a show of support for the embattled leadership, as Washington and its allies strongly criticised Moscow for violating Ukraine's sovereignty.

In what is Moscow's biggest confrontation with the West since the Cold War, Russian troops on Sunday tightened their grip on Crimea, the Ukrainian territory which has historic links to Russia and is home to Russia's Black Sea fleet.

"Russia chose this brazen act of aggression. If Russia wants to be a G8 country, it needs to behave like a G8 country," he continued.

"[Putin] may find himself with asset freezes on Russian business, American business may pull back, there may be a further tumble of the ruble," added Mr Kerry.

He said that Moscow still had a "right set of choices" to defuse the crisis. Otherwise, G8 countries and other nations were prepared to "to go to the hilt to isolate Russia".

The Kremlin's spokesman refused to respond to the remarks, saying the Kremlin had no comment "at the moment".

The US, Britain, France and Canada withdrew from preparations this week for a G8 summit in June in Sochi, the Russian winter Olympics city. President Barack Obama was due to speak to allies last night about the Ukraine crisis, said a White House spokesman.

William Hague, the Foreign Secretary, arrived in Kiev to meet the interim government installed by Ukraine's parliament after President Viktor Yanukovych fled last weekend after three months of tumultuous protest in the city's centre sparked by his decision to abandon a trade pact with the European Union in favour of a Russian bailout.

"The sovereignty and the territorial integrity of Ukraine has been violated and this cannot be the way to conduct international affairs," he said.

Ukraine has put its armed forces on combat readiness and issued a general call-up of the one million strong reserves to respond to Russia's seizure of the Crimea peninsula.

However, Kiev's small and under-equipped military is seen as no match for Russia's might.

The readiness of Ukraine armed forces was dramatically called into question after the government was forced to charge the head of the navy with treason after he defected to the renegade Crimea leadership.

Adm Denis Berezovsky announced he had switched allegiance to the pro-Russian authorities a day after he was appointed by interim president Oleksandr Turchynov.

"I swear to execute the orders of the [pro-Russia] commander-in-chief of the Autonomous Republic of Crimea," he said from the Crimean headquarters of the Russian Black Sea Fleet.

Ukraine insisted its fleet of 10 warships in the Crimean port of Sevastopol had not left the port and remained loyal to the government in Kiev.

Arseniy Yatsenyuk, the Ukrainian prime minister, warned that Kiev would request international intervention if the Russian military pressed its offensive in Crimea and potential elsewhere.

"This is actually a declaration of war to my country. We urge Putin to pull back his troops from this country and honour bilateral agreements," he said. "If he wants to be the president who started the war between two neighbouring and friendly countries, he has reached his target within a few inches."

Kiev has so far sought to avoid a blow-for-blow response to Russian moves despite the overnight call up. A marine infantry base in Crimea remained surrounded by Russian forces last night without a shot being fired.

Russian forces bloodlessly seized Crimea over the weekend, and yesterday surrounded several small Ukrainian military outposts and demanded the Ukrainian troops disarm. Some refused, leading to standoffs, although no shots were fired.

All eyes are now on whether Russia makes a military move in predominantly Russian-speaking eastern Ukraine, where pro-Moscow demonstrators have marched and raised Russian flags over public buildings in several cities in the last two days.

Revolutionary leaders yesterday addressed massive crowds in Kiev's Independence Square, the ground zero of the revolution. "Tens of thousands of people rallied in eastern and southern cities to show Ukraine is united and doesn't need any interference from another country," said Vitali Klitschko, the former boxing world champion-turned-presidential candidate.

For all Mr Kerry's strong language and the "ultimate" threats of sanctions, behind the scenes European and US diplomats were still working furiously to broker a face-saving end to the crisis.