A UN special envoy was forced to abandon a mission to Ukraine's Crimea region on Wednesday after being stopped by armed men and sheltering in a cafe, a reporter for Britain's ITN television said.

James Mates said envoy Robert Serry was trapped inside the coffee shop by a hostile pro-Russian crowd. When the envoy agreed to end his diplomatic mission, he was escorted to a car to the airport by police through a crowd shouting "Putin! Putin!"

"He was not kidnapped but he was seriously threatened," UN Deputy Secretary-General Jan Eliasson told reporters in New York by telephone from Kyiv, adding that "this action should be seriously condemned."

Eliasson said a group of unidentified men, some of them armed, surrounded Serry and said he should leave Crimea.

Crimea is under the control of Russian military forces, although Moscow describes its troops there who wear no insignia on their uniforms as "self-defence" units of the local administration.

Also on Wednesday, about 100 protesters waving Russian flags and chanting "Russia! Russia!" marched on a building used by international observers sent by the Organization for Security and Cooperation in Europe in Ukraine's Crimea region, a Reuters photographer on the scene said.