The UN envoy to Ukraine aborted his mission on Saturday, saying tensions made it impossible to visit Crimea as requested by secretary general Ban Ki-moon.
Robert Serry made the announcement as Russia's upper house of parliament debated a request from President Vladimir Putin to use military intervention in Ukraine.
After an emergency meeting of the UN Security Council on Friday, Ban ordered Serry to visit the Crimea in a bid to de-escalate tensions.
"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 on Saturday.
"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 on Saturday 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 on Friday.