RABAT (Reuters) – Morocco blamed Algeria on Saturday for a “serious and blatant” violation by the Polisario Front of an 18-year-long ceasefire in the disputed Western Sahara and urged the United Nations to intervene.

Some 1,400 supporters of the Algeria-backed Polisario Front independence movement, including foreigners, crossed the border from Algeria into a closed military zone where they uprooted barbed wire and fired shots in the air, the Foreign Ministry said in a statement.

It accused Algeria and the Polisario of trying to scuttle efforts to forge a peaceful solution to the conflict before a U.N. Security Council meeting on the dispute later this month.

Rabat and the Polisario Front have often accused one another of breaching the U.N.-supervised military truce.

But diplomats believe it is the first time in many years that Rabat has linked Algiers directly to an alleged violation.

They feared this would strain links between Algeria and Morocco, both of whose cooperation is seen by Western powers as crucial to the fight against al Qeada in north and sub-Saharan Africa and against illegal migration.

“…this action, which was initiated and carried out from Algerian territory, confirms the direct responsibility of this country in its preparation and implementation,” the Moroccan ministry said.

“This incident is (in line with) repeated attempts by Algeria and Polisario aimed at scuttling U.N. efforts to relaunch the dynamic of negotiations,” it added.

The ministry called on the United Nations to “assume responsibility and take the required actions.”

The dispute over Western Sahara, which is rich in phosphates and fish and may have offshore oil, has poisoned ties between Morocco and Algeria and blocked badly needed economic cooperation and growth in north Africa.

U.N.-brokered mediation has so far failed to break a deadlock over whether the territory should be an autonomous region of Morocco, as Rabat proposes, or have a referendum on independence, as Polisario wants.

Officials in Algiers and Polisario spokespeople were not immediately available to comment.

Morocco’s foreign ministry said an unspecified number of Polisario members and supporters were wounded when they stepped into a minefield and triggered a mine explosion.

The Algerian daily el Khabar said Saturday that at least three people were hurt and some 200 foreigners took part in the protest to back Polisario’s demand for an independent state.

Political sources in Rabat said one of those wounded was a Polisario member one of whose legs was severed by the explosion near Mahbes, one of the battlefields where Polisario guerrillas and Moroccan troops clashed in the 1980s.

(Reporting by Lamine Ghanmi; editing by Tim Pearce)