UNITED NATIONS (Reuters) - Morocco was considering “all options” if the United Nations does not address its accusations that the Polisario independence movement is threatening a 1991 ceasefire in the Western Sahara conflict, the foreign minister said on Wednesday.

Moroccan Foreign Minister Nasser Bourita addresses the 72nd United Nations General Assembly at U.N. headquarters in New York, U.S., September 20, 2017. REUTERS/Eduardo Munoz

Morocco claimed Western Sahara after colonial Spain left, but Polisario fought a guerrilla war for independence for the Sahrawi people until a U.N.-backed ceasefire, monitored by U.N. peacekeepers. The U.N. Security Council is due to renew the annual mandate for the peacekeeping mission later this month.

The region has effectively been split by an earthen wall separating an area controlled by Morocco that it claims as its southern provinces, and territory controlled by the Polisario with a U.N.-mandated buffer zone between them.

Morocco’s Foreign Minister Nasser Bourita met with U.N. Secretary-General Antonio Guterres on Wednesday to complain that the Polisario forces had entered the buffer zone and were building structures.

Bourita said he provided evidence to Guterres. He told reporters afterwards that “the U.N. should react to any threat to the ceasefire” and “today there is a real and very serious threat to that ceasefire.”

“Morocco is saying very clearly that all the options are under consideration,” Bourita said. “If the U.N., if the international community, don’t take their responsibilities, Morocco will take its own responsibility.”

In a letter to the U.N. Security Council on Monday, the Polisario rejected the “utterly unfounded and false allegations” made by Morocco.

U.N. spokesman Stephane Dujarric said on Monday that U.N. peacekeepers “have not observed any movement of military elements in the northeast territory” and that they are monitoring the situation closely.

U.N. talks have long failed to broker an agreement on how to decide on self-determination. Morocco wants an autonomy plan under Moroccan sovereignty. Polisario wants a U.N.-backed referendum including on the question of independence.

Tensions increased in Western Sahara in 2016 when Moroccan forces faced off with a brigade of Polisario forces in the remote Guerguerat area near the Mauritania border. Both sides withdrew their forces last year.

Relations between Morocco and the U.N. hit a low in 2016 after then-U.N. Secretary-General Ban Ki-moon used the word “occupation” to describe Morocco’s annexation of Western Sahara. Morocco expelled dozens of U.N. staff working for the mission there known as MINURSO.