The United Nations on Monday disputed claims by Morocco that the Polisario Front had carried out incursions in a buffer zone of Western Sahara, where a UN mission is monitoring a ceasefire.Morocco wrote to the Security Council on Sunday to report that fighters of the Polisario Front had entered the northeastern Western Sahara town of Mahbes in recent days in violation of a military agreement setting up the buffer zone.The fighters turned up in "military vehicles, and have set up tents, dug a ditch and built constructions using sandbags," Morocco's UN Ambassador Omar Hilale said in the letter, a copy of which was obtained by AFP.Asked about the Moroccan claims, UN spokesperson Stephane Dujarric said the MINURSO mission had "not observed any movement of military elements in the northeast territory.""MINURSO continues to monitor the situation closely," he said.Moroccan Foreign Minister Nasser Bourita has threatened a firm response to what he termed as "provocations" from the Algerian-backed Polisario, which is seeking independence for Western Sahara.The threats came as the Security Council is discussing a new mandate for MINURSO, which has been monitoring the truce since the end of a war in 1991.France is set to present a draft resolution on MINURSO and a vote is expected around April 25. Morocco has accused the Polisario of carrying out incursions near Guerguerat, another area of the buffer zone in the southwest near the Mauritanian border.In a report to the council, UN Secretary-General Antonio Guterres renewed a call for the Polisario to withdraw from Guerguerat and proposed that UN experts be dispatched to the tense area, but Morocco rejected that offer.Over the past year, MINURSO recorded five violations of the military agreement setting up the buffer zone by Moroccan forces and seven by the Polisario, the report said. Encouraging talks Despite tensions, Guterres said he was "encouraged" by his envoy's efforts to relaunch negotiations on a settlement that have been stalled since 2012.Former German president Horst Koehler, who was appointed envoy in August, has in recent months met with Morocco's King Mohammed VI, the presidents of Algeria and Mauritania, along with Polisario leaders and African Union officials.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. No date has been announced for re-starting talks, but diplomats do not rule out a return to the negotiating table later this year."The conflict over Western Sahara has lasted for too long and must be brought to an end for the sake and the dignity of the population of Western Sahara," Guterres wrote in the report obtained by AFP.A settlement would help shore up stability in the "wider region" of North Africa and the Sahel, "which is facing myriad political, economic and security challenges," he added in the report sent to the council last week.Morocco maintains that negotiations on a settlement should focus on its proposal for autonomy for Western Sahara and rejects the Polisario's insistence on an independence referendum.Morocco and the Polisario fought for control of Western Sahara from 1975 to 1991.