In an article published Thursday in Spanish newspaper El Pais, Dutch diplomat Peter Van Walsum said the latest extension of his contract had expired a week ago. U.N. officials in New York confirmed he was being replaced but declined to say at whose initiative he was leaving.

Since last year, Van Walsum, 74, has supervised four rounds of talks near New York between Morocco and the Polisario Front movement over the northwest African territory, which was annexed by Morocco after Spain withdrew from its former colony in 1975.

The talks have failed to break a deadlock over whether the resource-rich Sahara should be an autonomous region of Morocco, as Rabat proposes, or have a referendum of its people to decide whether or not it should be independent, as Polisario wants.

U.N. Secretary-General Ban Ki-moon has complained that the two sides are not negotiating, merely restating their starting positions.

Van Walsum, who took on his job three years ago, infuriated Polisario in April by saying in an assessment to the U.N. Security Council that he had concluded "that an independent Western Sahara was not a realistic proposition."

He later explained his remarks as a "gamble" to try to break the negotiating logjam.

The dispute over Western Sahara, which is rich in phosphates and fisheries and potentially has offshore oil, has poisoned relations between Morocco and Algeria -- which backs Polisario -- and held back north Africa's development.

The issue also has divided the Security Council, with France and the United States backing Morocco but South Africa and others favoring Polisario. Washington wants Sahara's status resolved so regional states can focus on what it sees as the more important question of combating terrorism.

Writing in El Pais, Van Walsum repeated his views, calling on Polisario to "consider a hypothetical solution other than total independence" and present its own autonomy proposal.

In New York, U.N. spokeswoman Marie Okabe told reporters an announcement of a new mediator would be made soon and that a fifth round of negotiations would be held in the fall.

"The secretary-general remains committed to offering the parties his good offices," she said. "He firmly believes that there is a need to revitalize the negotiation process in order to attain a peaceful solution in Western Sahara."

Polisario waged a low-level guerrilla war in Western Sahara from 1975 until 1991, when the United Nations brokered a ceasefire. The territory remains divided and many Sahara refugees live in camps in Algeria. (Editing by Bill Trott)