UNITED NATIONS — A senior United Nations diplomat who was sent to the Crimea region of southern Ukraine to assess the Russian military takeover there was threatened by armed men at gunpoint on Wednesday, and aborted his visit a day after it had begun.

The diplomat, Robert Serry, was confronted by a group of 10 to 15 gunmen as he left a meeting at a naval facility in Simferopol, the capital of the Crimea region, according to an account of the incident provided by Jan Eliasson, the United Nations deputy secretary general.

Mr. Eliasson, speaking to reporters by telephone from Kiev, the national capital, said that the gunmen confronted Mr. Serry and demanded that he go straight to the city airport and leave Crimea. Mr. Eliasson said that when he refused, they surrounded his car and threatened him, but he was allowed to enter a cafe and call Mr. Eliasson. The identities of the gunmen were not clear.

“He is in good shape physically,” Mr. Eliasson said. “He is not kidnapped.”

A journalist for the British network ITV, James Mates, who was traveling with Mr. Serry and witnessed the incident, reported that Mr. Serry was ending the trip. The United Nations later issued a statement saying “Robert Serry is taking a late flight out of Simferopol and will shortly return to Kiev to continue his mission, which was cut short by today’s incident.”