The head of the African Union has urged the United Nations to add human rights monitoring to the tasks of its peacekeeping mission in the disputed North African territory of Western Sahara.

The request came in a letter from African Union Commission chairwoman Nkosazana Dlamini-Zuma to U.N. Secretary-General Ban Ki-moon. A copy of the letter was seen by Reuters on Tuesday.

Zuma asked Ban to circulate the letter to the 15-nation Security Council, which is expected to vote on renewing the U.N. mission's (MINURSO) mandate this month.

Zuma stressed "the need to provide MINURSO with a human rights mandate, and in this regard the AUPSC (AU Peace and Security Council) has requested the U.N. High Commissioner for Human Rights to take appropriate steps to ensure sustained independent and impartial monitoring of human rights."

Ban's press office did not confirm receipt of the letter, which was dated March 30.

Morocco took control of most of the territory in 1975 when colonial power Spain withdrew, prompting the Polisario Front independence movement to wage a guerrilla war that lasted until 1991, when the U.N. brokered a ceasefire and sent in MINURSO.

Polisario now runs refugee camps at Tindouf in Algeria.

Zuma also expressed concern about the "illegal exploitation of Western Sahara's natural resources."

The 1991 ceasefire settlement called for a referendum to be held on the territory's fate. But the referendum, which MINURSO was deployed to help organize, never took place, and attempts to reach a lasting political deal have foundered.

The Security Council has come under increasing pressure in recent years to add human rights monitoring to MINURSO's mandate. Morocco opposes the idea and the council has never approved it.

France, traditionally an ally of Morocco, has been accused of supporting Rabat at the United Nations, but last year Paris rejected the allegation, saying Morocco was able to defend its interests without France's assistance.

Rabat wants Western Sahara to be an autonomous part of Morocco. Polisario, backed by a number of African countries including Algeria, wants to hold the long-promised referendum among the region's ethnic Sahrawis that would include the option of independence. Morocco and Polisario disagree on who should vote in a referendum.

Western Sahara is slightly bigger than Britain and has a population of under 500,000. It is rich in phosphates, used in fertilizer, and potentially, offshore oil and gas. Polisario has complained about Western companies searching for natural resources based on permits from Morocco.