A confidential U.N. document says Morocco violated a 1991 cease-fire agreement with the Polisario Front independence movement by sending armed security personnel and equipment into the contested Western Sahara region without prior notice to U.N. peacekeepers.

The note to the U.N. Security Council from the Department of Peacekeeping Operations, obtained Monday by The Associated Press, said the Polisario Front deployed 32 armed military personnel in response, also in violation of the cease-fire.

Secretary-General Ban Ki-moon expressed deep concern Sunday at "the tense situation that has developed in the narrow buffer strip in southwestern Western Sahara between the Moroccan berm and the Mauritanian border," his spokesman said.

The U.N. chief said this was the result of "the introduction of armed units from Morocco and the Polisario in close proximity to each other" and urged both sides to respect "the letter and the spirit of the cease-fire agreement," the spokesman said.

Morocco annexed Western Sahara, a former Spanish colony, in 1975 and fought the Polisario Front. The U.N. brokered a cease-fire in 1991 and established a peace-keeping force known by the acronym MINURSO to monitor it and help prepare a referendum on the territory's future, which has never taken place.

Morocco considers Western Sahara its "southern provinces" and has proposed wide-ranging autonomy, but the Polisario Front insists on self-determination through a referendum.

Morocco expelled more than 70 U.N. civilian staffers with MINURSO in March to protest the secretary-general's perceived gaffe in using the word "occupation" to describe Morocco's annexation of Western Sahara. Twenty-five staffers returned last month but MINURSO is still not fully operational.

The confidential note said MINURSO confirmed through multiple ground and air patrols over the Aug. 16-25 period that Morocco "was conducting what it maintained to be clearance of damaged vehicles in order to reduce the capacity of smugglers to operate, along with road laying activities" in the southern part of Western Sahara.

"This was initiated without prior notice to MINURSO, contrary to requirements specified in Military Agreement No. 1" of the cease-fire, the note said, "and with the support of armed security personnel from the Royal Moroccan Gendarmerie."

It said Morocco confirmed these activities on Aug. 18, and in response MINURSO significantly reinforced its monitoring activities in the area and is carrying out extensive consultations with both sides to de-escalate the situation and avert a resumption of hostilities.

The Polisario Front sent a letter to the U.N. secretary-general on Aug. 15 urging him to end Morocco's violation of the cease-fire agreement.

Polisario Minister of Defence Abdallahi Lehbib Belal said in a letter to MINURSO's chief of staff on Sunday that he deployed military forces to the area on Sunday morning "to prevent further Moroccan activities beyond the berm, including the asphalted road it has started to build."

The Polisario Front has said that if the road is built Morocco would have to maintain it, which would give its military a permanent presence in the restricted area.

A Moroccan Foreign Ministry official told The Associated Press on Monday that "it is the Polisario that has been engaging in provocation" — not Morocco.

"We launched our anti-smuggling campaign in co-operation with Mauritania, then the Polisario began sending in troops this past Friday night/Saturday morning near the border," the official said, speaking on condition of anonymity because he was not authorized to speak publicly.

The U.N. peacekeeping department said in the confidential note that Morocco informed MINURSO on Sunday that it observed 20 Polisario Front "elements" in the buffer strip. It said the Polisario Front told MINURSO later Sunday it was establishing a checkpoint "to stop the Moroccan construction works" — but not civilian traffic.

The Polisario Front was informed by MINURSO that the deployment of its forces in the buffer strip was a violation of the cease-fire agreement, the note said.

It said MINURSO plans to set up a special working group to review the potential violations by both sides.

News from © The Associated Press, 2016