Morocco has launched a military operation in a United Nations-patrolled buffer strip in Western Sahara, escalating tensions in the contested territory and threatening a fragile truce that has held in the region for almost three decades.

Authorities in Morocco said on Friday that their forces had moved into the buffer strip, at the Guerguerat zone, following weeks of “provocations” from members of the Polisario Front, a pro-independence movement.

Since Oct. 21, the Polisario Front has blocked the movement of goods and people along the key road in area connecting Morocco and Mauritania, the Ministry of Foreign Affairs said in a statement. The ministry accused the group of carrying out “acts of banditry” and “harassing” U.N. peacekeeping forces operating in the region.

Tensions in the region date to 1975 when Morocco annexed the desert, a former Spanish protectorate that was also briefly occupied by Mauritania. For years, the Polisario fought for independence from Morocco in an armed resistance that lasted until 1991, when the United Nations negotiated an armistice between the two parties.