UNITED NATIONS (AP) — U.N. Secretary-General Antonio Guterres ramped up efforts Friday to try to get Morocco and pro-independence supporters in disputed Western Sahara to step back from a renewed flare up of fighting, warning that the clashes could rupture a nearly 30-year cease-fire and have “grave consequences.”

In recent days, Guterres and other U.N. officials have been working the phones and been involved in “multiple initiatives to avoid an escalation” -- so far unsuccessfully, U.N. spokesman Stephane Dujarric said.

But “the secretary-general remains committed to doing his utmost to avoid the collapse of the cease-fire that has been in place since Sept. 6, 1991 and he is determined to do everything possible to remove all obstacles to the resumption of the peace process,” Dujarric said.

The Moroccan military launched an operation in the U.N.-patrolled Guerguerat border zone to clear a key road it said had been blockaded for weeks by supporters of the pro-independence Polisario Front.

A Polisario envoy accused the Moroccan military of firing at innocent protesters and said that led to clashes Friday between Moroccan and Polisario forces. The envoy urged the United Nations to intervene.

Morocco annexed Western Sahara, a former Spanish colony believed to have considerable offshore oil deposits and mineral resources, in 1975. The Polisario Front fought for independence and the U.N. brokered the 1991 cease-fire and established a peacekeeping mission to monitor the truce and help prepare a referendum on the territory’s future. It has never taken place.

Morocco has proposed wide-ranging autonomy for Western Sahara. But the Polisario Front insists the local population, which it estimates at 350,000 to 500,000, has the right to a referendum.

Dujarric said the United Nations has been “expressing our concern about the situation in Guerguerat for quite some time.”

“We have seen over the last few weeks violations from both sides,” he replied when asked who was responsible for the latest fighting. “We have condemned, and we condemn all violations of the cease-fire.”

Dujarric said a special civilian-military team from the U.N. peacekeeping mission, known as MINURSO, has been on the ground in the Guerguerat area “since the beginning of the crisis, and we’ve had military observers there also remain overnight.”

Friday’s action came after the Polisario Front reportedly suggested earlier this week it would reconsider its engagement in the U.N.-led political process on Western Sahara's future, threatening to withdraw from the cease-fire if any Moroccan civilian or military personnel entered the buffer zone.

Moroccan forces set up a security cordon overnight in the Guerguerat buffer zone on Western Sahara’s southern border with Mauritania “in order to secure the flow of goods and people through this axis,” the General Staff of the Moroccan Royal Armed Forces said in a statement Friday.

It said they intervened because about 60 people supervised by Polisario were blocking a road connecting Morocco with Mauritania, and called it a “non-offensive operation” that would involve use of arms “only in the case of self-defense.” The Moroccan Foreign Ministry said the road has been blocked for more than three weeks.

The Polisario’s ambassador to Algeria, Abdelkader Omar, said Moroccan forces “opened fire on innocent civilian protesters” and Polisario fighters came to the protesters’ defense, prompting “intense clashes” Friday.

Speaking on Algerian television network El Bilad, Omar said, “It’s the U.N.’s duty to intervene urgently to stop this aggression against the Sahrawi people.”

Polisario chief Ibrahim Ghali sent an urgent letter to the U.N. secretary general and U.N. Security Council about the intervention.

The letter to Guterres, obtained by The Associated Press, accused Moroccan forces of launching “a brutal attack on unarmed Sahrawi civilians who have been demonstrating peacefully in Guerguerat.”

Ghali called it “an act of aggression and a flagrant violation of the cease-fire” that the U.N. Security Council should condemn in the strongest terms.

He told Guterres the timing of Morocco’s action -- on the eve of an “engagement” between the U.N. chief and the Polisario -- “demonstrates clearly that the action is a premeditated act of aggression ... to torpedo your efforts to diffuse tensions and de-escalate the situation in Guerguerat.”

By launching the military operation, Ghali said, Morocco has “seriously undermined” not only the cease-fire “but also any chances of achieving a peaceful and lasting solution to the decolonization question of Western Sahara.”

Dujarric, the U.N. spokesman, said he hadn’t seen the letter but confirmed the secretary-general “has been making calls to the different parties, and his people on the ground have been involved.”

Guterres has not had a personal envoy in Western Sahara since former German president Horst Köhler left the post in May 2019 for health reasons.

Dujarric said the secretary-general shared widespread “frustration” that a successor -- who requires approval from Morocco, the Polisario and the Security Council -- has not be appointed after 18 months, stressing that “it’s not been for a lack of trying.”