Pulled back from the brink of civil war, Kiev has warned that it is on the brink of international war as Vladimir Putin ignored all warnings about consequences of military action on Ukrainian soil.
In a tense 90-minute telephone conversation last night US president Barack Obama warned Putin that Russia had flouted international law and urged him to pull forces back to bases in Crimea.
But there was no convincing the Kremlin.
Putin spokesman said he had "underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory".
Many of Crimea's residents, majority ethnic Russians, have welcomed the 6,000 troops now patrolling the street. Ukraine's east has generally opposed the Kiev revolutionaries, who ousted pro-Moscow president Viktor Yanukovych last week.
The United States will "suspend upcoming participation in preparatory meetings for the G8. Going forward, Russia’s continued violation of international law will lead to greater political and economic isolation," the White House said in an emailed statement.
Ukraine says it has put its army on full combat alert following the widespread deployment of Russian troops.
Ukraine has asked Nato to look at all ways it can help the country protect its borders, foreign minister Sergei Deshchiritsya told reporters on Sunday. Nato, and EU officials, are both due to hold further talks in the coming days.
In a second emergency meeting of the United Nations' Security Council last night Ukraine's UN ambassador asked for it to do everything possible to stop Russian "aggression".
UN secretary general Ban Ki-moon urged Putin in a phone call to "urgently engage in direct dialogue with the authorities" in Kiev.
British ambassador Sir Mark Lyall Grant, who called the UNSC meeting, said the UK had sought "an immediate and full explanation" for Russia's decision to authorise military action and for its basis under international law.
He told the Security Council: "The United Kingdom is deeply concerned by the escalation of tensions in the Crimean Peninsula, and by the fact that the Russian parliament has authorised Russian military action on Ukrainian soil against the wishes of the Ukrainian government.
"This action is a grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine."
He added: "It is critical that the Russian Federation respects the sovereignty, independence and territorial integrity of Ukraine and takes immediate steps to calm this dangerous situation."
Speaking after the meeting, Sir Mark said that "there is no justification for Russia's military activities in the last 48 hours".
Canada has recalled its ambassador to Moscow for consultations and said it is also preparing to withdraw from the G8 summit in Sochi, Russia.
Canadian Prime Minister Stephen Harper said he was also suspending Canada's preparations for a G8 summit in the Russian resort of Sochi in June.
Hague summoned the Russian ambassador Alexander Yakovenko to the Foreign Office last night to meet political director Simon Gass and hear the UK's reaction to the deepening crisis.
An FO spokesman said: "The political director expressed deep concern at the Russian Parliament's decision to authorise military action in Ukraine against the wishes of the Ukrainian government. The political director asked the ambassador to urge his government to respect the sovereignty, independence and territorial integrity of Ukraine."
Hague, who will fly to Kiev for crisis talks on Sundy, said he was "deeply concerned" at the decision of the Russian parliament to back Vladimir Putin's call for the use of troops in Ukraine to protect ethnic Russians and a military base in Crimea.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine," he said.