The Organization for Security and Co-operation in Europe (OSCE) says it is sending a mission of 35 military observers to Ukraine.
The unarmed observers, from 18 of the organisation's 57 members, were on their way to Odessa, in southern Ukraine.
The observation mission, which is scheduled to last until March 12, had been requested by the new pro-Western authorities in Kiev.
"It is my hope that this military visit will help to de-escalate tensions in Ukraine," OSCE Secretary General Lamberto Zannier said in a statement.
"By providing an objective assessment of the facts on the ground, the OSCE will be better placed to foster a political solution to the current crisis through dialogue," Zannier said.
Pro-Russian forces have taken military control of the Ukrainian peninsula of Crimea, sparking what is being described as the worst East-West stand-off since the Cold War.
Among the OSCE countries contributing military observers to the mission are the United States, France, Germany, Canada and the United Kingdom.
Military observers are unarmed, and are tasked with finding out facts on military matters. They report back to OSCE participating states.
"There have been roughly 90 such missions over the past 15 years. They happen on a very regular basis," OSCE spokeswoman Natacha Rajakovic said.
Washington has called for a full-scale OSCE monitoring mission in order to ease the crisis, but the organisation's member states failed this week to agree, in the face of Russian objections.
Russia's envoy to the OSCE, Andrei Kelin, told reporters on Monday that earlier missions in breakaway Serbian and Georgian territories Kosovo and South Ossetia had "aggravated the situation".
Washington's top envoy in Europe, Victoria Nuland, said a monitoring mission would allow Russia in Crimea to "pull its forces back to base and have them replaced by independent monitors".
