U.N. Deputy Secretary-General Jan Eliasson was flying Sunday to Ukraine, where a threatened Russian invasion has risked sparking the worst crisis since the Cold War.
The world body's Secretary-General Ban Ki-moon ordered Eliasson to visit Ukraine after two emergency sessions of the U.N. Security Council that saw Western countries trade blame with Russia for the unfolding crisis in Ukraine.
"The deputy secretary-general will be personally apprised of the facts on the ground and will subsequently brief the secretary-general on the next steps the United Nations could take to support the de-escalation of the situation," Ban's spokesman said.
Ukraine warned it was on the brink of disaster and called up military reservists as world leaders huddled for urgent consultations cross the globe.
Kiev appears on the brink of losing control of the Russian-leaning Crimean Peninsula with the defection of its navy commander to pro-Kremlin forces.
Ukrainian troops are in a tense standoff, with hundreds of armed men believed to be under Russia's orders surrounding their military base in Crimea and demanding their surrender.
Russia's parliament voted Saturday to allow President Vladimir Putin to send troops into the ex-Soviet state -- a decision condemned by the United States as a "violation of Ukrainian sovereignty."
US Secretary of State John Kerry will also visit Kiev for talks on Tuesday that US officials said would lend support to Ukraine's interim Western-allied leaders.