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GENEVA (Reuters) - The U.N. envoy for the contested Western Sahara will convene talks in Switzerland later this week, seeking compromise after a first round in December that marked a return to the negotiating table for the first time in six years, a statement said.

United Nations efforts have repeatedly failed to broker a settlement over the desert territory, contested between Morocco and the Algerian-backed Polisario since Spain left in 1974.

U.N. envoy Horst Koehler said after talks in December that all sides had agreed to meet again in early 2019.

He has invited delegations from Morocco, the Frente Polisario, Algeria and Mauritania to the meeting in Switzerland set for March 21-22, a U.N. statement said.

“The meeting will take place in accordance with UN Security Council resolution 2440 as a further step in the political process toward reaching a just, lasting, and mutually acceptable political solution, which will provide for the self-determination of the people of Western Sahara,” it said.

“The purpose of the meeting is for delegations to start approaching elements needed for building an enduring solution based on compromise,” it added.

Morocco has offered autonomy to Western Sahara, a thinly populated region that has rich fishing waters and phosphate deposits, and may also have oil and gas reserves.

The Polisario, which waged a low-intensity guerrilla war until a ceasefire in 1991, reject this and want a referendum, with independence for Western Sahara as one option.