UN envoy Robert Serry was detained briefly by unidentified gunmen in Simferopol, the capital of Crimea, a spokesman for Ukraine's foreign ministry says.
"His vehicle was blocked in Simferopol by unknown gunmen," Yevgen Perebyinis told AFP on Wednesday.
"They tried to take him to the airport by force but he refused."
The United Nations in New York said he was threatened by gunmen but denied he had been kidnapped, as reported by some media.
One journalist who said he was with him tweeted that he was held in a coffee shop, with men outside preventing him from leaving.
"UN special advisor Robert Serry had been visiting navy commander when his car was blocked. Stand off followed," said James Mates, a reporter for British channel ITV.
"He refused to go with men blocking car, got out and walked until he found coffee shop. He's asked ITV News team to stay with him."
Serry had been sent to the Black Sea peninsula - which is under the de facto control of pro-Russian forces - to "take stock of the situation", UN's deputy secretary-general Jan Eliasson told journalists in Kiev earlier, before the news broke.