RABAT (Reuters) - Morocco has finished building a sand barrier in a U.N.-monitored buffer zone in Western Sahara, Prime Minister Saad Eddine El Otmani told Reuters on Tuesday, after the Polisario Front independence movement withdrew from a ceasefire. The Moroccan army entered the buffer zone on Friday to open a road linking Western Sahara with Mauritania which had been blocked by Polisario supporters and fighters, leading the group to quit the 29-year-old truce agreement. Speaking in an interview with Reuters, El Otmani reiterated that Morocco was sticking to the ceasefire and said there had been only “skirmishes and sporadic fighting” in recent days as concerns grew that a long-frozen conflict could reignite. The Algeria-backed Polisario says it has repeatedly bombarded Moroccan positions on the sand wall that Rabat built in the 1980s along much of the frontier running for hundreds of miles through the desert.