Mohamed Abdelaziz, the head and co-founder of the Polisario Front independence movement in the Western Sahara, died May 31. He was in his late 60s.

The group — based in Tindouf, in southern Algeria — announced the death but did not cite a cause or place.

The death of Mr. Abdelaziz, leader of the Polisario Front for four decades, comes at a time of growing tensions over the fate of the Western Sahara. The Polisario Front has fought for four decades for independence for the vast, mineral-rich territory on Africa’s Atlantic coast, which was annexed by Morocco after Spain withdrew in 1975.

Morocco considers the territory its “southern provinces” and has pumped funds into the area’s development over the years. Morocco is also becoming increasingly assertive on the topic with the United Nations, which has worked for years to help settle the issue of Western Sahara’s status.

Mr. Abdelaziz was born in Smara, which is now in Moroccan-controlled Western Sahara, and led the Polisario Front, which he helped found, since 1976, according to Algeria’s state-run APS news agency.

Mohamed Abdelaziz in 2015. (Francisco Seco/AP)

The issue of Western Sahara has prompted new friction between two North African neighbors, Morocco and Algeria. Algeria supports the Polisario Front, and like numerous other African countries recognizes the Saharawi Arab Democratic Republic it defends.

The Moroccan government has proposed wide-ranging autonomy for the region, but the Polisario Front insists on self-determination through a referendum for the local population — as called for in U.N. resolutions.

Morocco expelled most U.N. civilian staff in April after U.N. Secretary-General Ban Ki-moon used the word “occupation” to refer to the situation after a March visit to a camp for Western Sahara refugees in southern Algeria during which he met with Mr. Abdelaziz.

In April, a top member of the Polisario Front, Bachir Mustafa Sayed, warned that war is possible over the disputed territory if the U.N. Security Council fails to set a timetable for a vote on self-determination.

Mr. Abdelaziz warned in a letter to Ban that Morocco will have “a green light to a military aggression” unless the Security Council imposes “real and direct pressure” on Morocco to restore the U.N. mission’s work. He warned that the Saharawi people in Western Sahara will defend their rights in the face of aggression “by all legitimate means, including armed struggle.”

The Polisario had mounted a desert war against Morocco after the territory was annexed.

Ban called Mr. Abdelaziz “a central figure in the search for a resolution of the Western Sahara conflict,” according to a statement released by the U.N. spokesperson. Ban said he “looks forward to continuing to work to help the parties to achieve a mutually acceptable political solution” providing for the self-determination of the people of Western Sahara.

Issandr El Amrani, North Africa project director with the International Crisis Group, said the impact of Mr. Abdelaziz’s death on the U.N. contingent was unclear “but it adds uncertainty at a time when the mission is under pressure.”

Amrani noted the saber-rattling tone among some Polisario activists in the absence of a diplomatic breakthrough. “The next Polisario leader will have to contend with that sentiment,” Amrani said.

The Polisario Front has appointed the head of the movement’s National Council, Khatri Adduh, as interim Polisario leader.

In the refugee camps of Tindouf, Saharawi blogger Mohamedsalem Werad described the overall atmosphere on Tuesday as quiet, sad and uncertain. “Many people are expecting all kinds of scenarios,” he said.

— Associated Press