The UN Security Council will meet today for a second round of emergency consultations, officials said, after Russia's parliament approved the deployment of troops to Ukraine.
The president of the Security Council, currently Luxembourg, invited members to "informal consultations" at 1900 GMT, a statement said.
The announcement came just hours after Russian leader Vladimir Putin won approval from lawmakers to send Russian troops into Ukrainian territory.
Britain's ambassador, Mark Lyall Grant, tweeted that the meeting was called at London's request.
The UN envoy to Ukraine, Robert Serry, announced earlier that he was leaving the country because it was impossible to visit Crimea as requested by Secretary General Ban Ki-moon.
Ban ordered Serry to visit the Crimea in a bid to de-escalate tensions after an emergency meeting of the UN Security Council yesterday.
"I have since been in touch with the authorities of the autonomous republic of Crimea and have come to the conclusion that a visit to Crimea today is not possible," Serry said in a statement from Kiev.
"I will therefore proceed to Geneva, where I will tomorrow brief the secretary general on my mission and consult with him on next steps," he added.
Ukrainian Defence Minister Igor Tenyukh said today that Russia had sent 30 armoured personnel carriers and 6,000 additional troops into Crimea.
Dozens of pro-Russian armed men in full combat gear patrolled outside the seat of power in Crimea's capital Simferopol.
Similar gunmen seized the city's parliament and government buildings on Thursday and took control of its airport and a nearby military base yesterday.
