Tindouf - Behind a long sand wall winding through the disputed Western Sahara, leaders battling for the independence of the former Spanish colony say they are on alert.Morocco insists the territory is an integral part of its kingdom, but the Algeria-backed Polisario Front demands a referendum on self-determination."There are 25 000 Sahrawi fighters and any other Sahrawi is recruitable," whether man or woman, says Polisario defence chief Abdullahi Lehbib.His comments come after the president of the Sahrawi Arab Democratic Republic (SADR), Brahim Ghali, on Sunday said "all options remain open" to resolve the dispute, hinting at a possible return to armed struggle.Morocco fought the Polisario from 1975 to 1991, gaining control of most of the territory before a UN-brokered ceasefire took effect.It has built six mostly sand barriers along roughly 2 700km to cordon off this land.On the other side is the Polisario-run SADR.Since the 1991 ceasefire, the United Nations has maintained peace keepers in the vast desert territory where around half a million people live.A referendum on independence was set for 1992 but was aborted when Morocco objected to the proposed electoral register, saying it was biased."Despite the ceasefire we have continued recruitment and conscription," Lehbib tells AFP.Tensions flared last year after the Polisario set up a new military post in the district of Guerguerat near the Mauritanian border, within a stone's throw of Moroccan soldiers.The move came after Morocco last summer started building a tarmac road in the area south of the buffer zone separating the two sides. 'Resistance Museum' "The stalling of the peace process, and especially the situation in Guerguerat, mean that we are on alert along the wall," Lehbib says.Trenches, barbed wire and mine fields flank the Sahrawi side of the wall near El-Mahbes.Cheikh Bechri Mhame, the sector's commanding officer, describes how his men patrol the area in four-wheel-drives, looking out for any movement near the barrier which is two to three metres high.Dressed in military fatigues and armed with automatic rifles, his soldiers are also tasked with cordoning off land peppered with anti-personnel mines.Around a hundred kilometres from there, in one of the refugee camps in the Tindouf area in southwest Algeria, the director of the "Resistance Museum" tells visitors the army was able to adapt to life along the wall.Built between 1980 and 1987, Morocco's "defence wall wasn't efficient in defending the Moroccans," Mohamed Ouleda says."The (Sahrawi) army chose certain spots to carry out incursions, even if at the time there were only 12 000 fighters to face a Moroccan armada."He said Sahrawis captured "511 Moroccan prisoners of war between the wall's construction and the ceasefire", citing Red Crescent documents that AFP was not able to obtain.In his museum, Ouleda shows off "spoils of war... retrieved on the other side of the wall": weapons, armoured vehicles and Moroccan military documents.Polisario chief Ghali says the wall's construction in fact led to a "war of attrition" for the Moroccans, not the Sahrawis."The Moroccans built the wall thinking it was insurmountable," he says."But it became an economic, psychological and moral burden for the Moroccan army instead of a solution.""That's when Morocco started the negotiations that led to the UN peace plan and ceasefire."