By William Maclean

ALGIERS, May 31 (Reuters) - Failure to break the long deadlock over Western Sahara in peace talks starting next month could destabilise north Africa and reignite Polisario's armed struggle, the independence movement's leader said on Thursday.

Polisario leader Mohamed Abdelaziz told Reuters the movement was under "tremendous pressure" to get results from its supporters who were frustrated at Morocco's refusal to let them vote in a referendum on self determination.

"We prefer a peaceful solution ... but if they close all avenues and they persist in trying to impose a fait acompli we will try to use all means to defend ourselves," Abdelaziz said.

Morocco and Polisario are expected to hold U.N.-sponsored talks in June to try to negotiate an end to Africa's oldest territorial dispute by settling the status of the former Spanish colony, annexed by Morocco after Madrid pulled out in 1975.

Morocco wants talks about self-rule for the territory under Moroccan sovereignty, but Algeria-backed Polisario has demanded a referendum that would include the option of full independence.

Abdelaziz, who is also president of the self-proclaimed government in exile for Western Sahara, said he expected the talks to start at the United Nations on June 18.

In a peace proposal submitted to the United Nations in April, Polisario said it was ready to negotiate with Morocco on ways to hold a vote that would offer a choice between independence, integration into Morocco and self-governance.

He added: "If Morocco respects this decision and demonstrates a political will on the next round of negotiations I think we can say real options for peace are there."

"But if they continue in their intransigent attitude and lack of will demonstrated since the death of King Hassan II, I think they are pushing us back to square one and pushing the entire region to square one. That could open the entire region to any kind of danger including the armed struggle."

TERRITORY

Morocco, claiming centuries-old rights over the territory, annexed it in 1975 when Spain withdrew, triggering a low-level guerrilla war with the Polisario.

A U.N. ceasefire agreement in 1991 promised a referendum on the fate of the territory, but it never took place and Rabat now rules it out, saying autonomy is the most it will offer.

No country recognises Morocco's sovereignty over Western Sahara. Many countries have extended ties to Polisario and recognise its claim to a Sahrawi Arab Democratic Republic.

Morocco says independence is not possible because Sahrawis are scattered across Algeria, Mali, Mauritania and Morocco.

The United States has become impatient for a deal in hopes it will bring more cooperation between North African states and help combat terrorist groups in the regions bordering the Sahara.

The Western Sahara dispute is the main cause of tension between Morocco and Algeria, whose land borders, closed in 1994 amid security tensions, remain shut.

Abdelaziz said 16 years after the ceasefire Sahrawis were intensely impatient for the promised vote on self-determination. "It is very difficult for the political leadership to keep convincing them just to wait another year," he said.