The planned congress in the disputed territory threatens the stability of the Maghreb and next month's talks between the two sides, the Moroccan government said in a letter to the U.N. secretary general published in Moroccan state media.

Polisario's national secretariat will put the proposal to prepare for war to a vote at a congress to be held on December 14-16 in the Polisario-controlled outpost of Tifariti, Polisario official Mohamed Beissat said in a Reuters interview on Monday.

If adopted, it would be the first time in 16 years that preparations for war had been part of Polisario strategy.

"Morocco asks the U.N. Secretary-General (Ban Ki-moon) to take the necessary steps to face these dangerous and provocative manoeuvres that threaten peace and stability in the region," the Moroccan letter said.

"These dangerous and provocative machinations are contrary to a climate of trust and serenity which must prevail at this crucial moment of the process of searching a negotiated political solution to this regional dispute," it added.

When Spain withdrew from its then colony in 1975, Morocco annexed the territory and fought Polisario in a low-level war for 16 years. A U.N. ceasefire accord in 1991 promised a referendum on the future of the mineral-rich territory.

The plebiscite never took place and Morocco now rules it out, saying autonomy is the most it will offer.

Rabat said holding a Polisario congress in Tifariti would violate the U.N. ceasefire agreement, which it says made Tifariti part of a buffer zone between Algeria-backed Polisario guerrillas and Moroccan troops.

"All activities undertaken in this buffer zone are completely illegal..." the letter said.

The upper and lower houses of the Moroccan parliament held a joint meeting on Wednesday to denounce the Polisario meeting.

Polisario holds a congress every three to four years. The last one was in 2003. No country officially recognises Morocco's rule over Western Sahara and the U.N. Security Council is divided over a solution.

Morocco and Polisario held two rounds of U.N.-sponsored peace talks this year aimed at breaking the stalemate. A third round is due on January 7-9.

Rabat says it has a French-backed plan offering limited autonomy for Western Sahara but Polisario proposes a referendum with independence as one option.