The West is facing its biggest confrontation with Russia since the cold war, with Ukraine mobilising for war and calling up its reserves after Russian President Vladimir Putin declared the right to invade his neighbour.
Ukraine's new prime minister, Arseniy Yatsenyuk, 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."
Watch: Ukraine mobilises army after Russia's threat to invade
Ukraine's defence ministry was ordered to conduct the call-up, potentially of all men up to 40 in a country that still has universal male conscription.
China's Foreign Ministry said it was "deeply concerned" by what was happening in Ukraine. China has so far shown little public interest in participating in any financial aid for Ukraine, or getting involved diplomatically, in line with the low-key approach it takes to many international crises.
"China will keep closely monitoring developments, and calls on all sides to find a political resolution via dialogue and negotiations," the ministry said.
Russian forces who have already bloodlessly seized Crimea - an isolated Black Sea peninsula where most of the population is ethnic Russian and where Moscow has a naval base - yesterday tried to disarm the small Ukrainian contingents there. Some Ukrainian commanders refused to give up weapons and bases were surrounded.
Putin's declaration he had the right to invade his neighbour - for which he quickly received parliament's approval - resulted in a 90-minute phone call between President Barack Obama and Putin, the kind of direct confrontation between the men rarely seen since the end of the cold war.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House said after the leaders spoke on Saturday.
A statement posted on Putin's website said that in his conversation with Obama, he defended his actions, citing "the provocative and criminal actions on the part of ultra-nationalists who are in fact being supported by the current authorities in Kiev".
Ukraine Foreign Minister Andriy Deshchytsya said he sent a request to Nato to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine".
Ukraine also appealed for help to Britain and the United States, as co-signatories with Moscow to a 1994 accord guaranteeing Ukraine's security after the breakup of the Soviet Union.
Washington and its allies have suspended preparations ahead of a G8 summit in Sochi, where Putin had just finished staging his US$50 billion winter Olympic Games. But the US and other Western governments have few options to counter Russia's military moves.
US Secretary of State John Kerry, interviewed on news shows yesterday, raised the possibility of boycotting the summit.
British Foreign Secretary William Hague, meanwhile, left for Kiev to meet Ukraine's new leaders.
At Kiev's Independence Square, thousands demonstrated against Russian military action. Speakers delivered rousing orations and placards read: "Putin, hands off Ukraine!"
So far there has been no sign of Russian military action outside Crimea, but Kiev officials accused Moscow of being behind the pattern of violent protests in eastern cities.
Additional reporting by Associated Press, Agence France-Presse