Russia's G8 partners have condemned Moscow's military build-up in Ukraine amid fresh diplomatic efforts to avert a dangerous escalation of the crisis.
The world's seven major industrialised powers also suspended preparations for the G8 summit in Sochi in June.
Meanwhile, European Union foreign ministers are due to meet in emergency session in Brussels.
The moves come as Russian military forces continue to strengthen their grip on the Crimean peninsula.
Ukraine's interim government has accused Russia of having declared war, and has ordered the mobilisation of its armed forces.
But Russian President Vladimir Putin has so far defied calls from the West to pull back his troops.
He insists Russia has a right to protect its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine.
The UN said on Sunday that Deputy Secretary-General Jan Eliasson was travelling to Ukraine to be 'personally apprised of the facts on the ground'.
A statement said he would brief UN Secretary General Ban Ki-moon 'on the next steps the United Nations could take to support the de-escalation of the situation'.
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The G7 urged Russia to hold talks with Ukraine to address any human rights or security concerns it has.
In a statement released from the White House it said: 'We, the leaders of Canada, France, Germany, Italy, Japan, the United Kingdom and the Uni.