UNITED NATIONS (Reuters) -- The Polisario Front, the Western Sahara independence movement, on Friday rejected in advance a plan by Morocco to propose to the United Nations that the desert territory be made an autonomous part of Morocco.

Only holding of a referendum on independence "offers real possibilities for a just and lasting resolution of the conflict of Western Sahara," Polisario representative Ahmed Boukhari said in a letter to U.S. Ambassador John Bolton, the UN Security Council president for February.

The northwest African territory of about 260,000 people has become home to the continent's longest-running territorial dispute since it was seized by Morocco after gaining independence from Spain in 1975.

That triggered a low-intensity guerrilla war between the Polisario Front and Rabat that simmered on until 1991, when the United Nations brokered a cease-fire and sent in peacekeepers in anticipation of a referendum to decide the territory's fate.

But attempts at a referendum have come to nothing despite repeated Security Council resolutions pressing Morocco and the Polisario to resolve their three-decade impasse.

The main obstacle has been Morocco's insistence that independence could not be an option, even as the Polisario Front insisted that it had to be.

Moroccan officials then announced last month that Rabat would put forward in April a formal plan for autonomy for the area, which is rich in phosphates and fisheries and may also have offshore oil deposits.

While Rabat has repeatedly promised to propose a plan to end the deadlock, it has never set a date for unveiling it.