Morocco on Wednesday began work in an area of the disputed Western Sahara at the centre of a crisis that pits the kingdom against the pro-independence Polisario Front.

Heavy machinery was seen at work in the buffer zone at Guerguerat along the sandy track that leads to the border with Mauritania, the correspondent said, while goods trucks passed in both directions.

The crisis erupted after Morocco launched a military operation Friday to reopen a key highway at the Guerguerat border crossing between Western Sahara and Mauritania.

It accused the Polisario of blocking the highway, which is key to trade with the rest of Africa.

The Algerian-backed Polisario, which does not recognise the existence of the highway, responded by declaring the end of an almost three-decade UN-supervised ceasefire in Western Sahara.

Moroccan Prime Minister Saad-Eddine El Othmani said Tuesday that the extension of a barrier reaching the border had been completed.

A 2,700-kilometre (1,700-mile) Moroccan defensive wall cuts through Western Sahara.

El Othmani told the Reuters news agency that the aim of extending the sand barrier to the frontier was to secure "once and for all civilian and commercial traffic in Guerguerat between Morocco and Mauritania".

Rabat controls around three-quarters of Western Sahara, a vast swathe of desert on the Atlantic coast, including its phosphate deposits and its lucrative ocean fisheries. The Polisario controls the rest.

Morocco says Western Sahara is an integral part of the kingdom and has offered autonomy for the disputed territory, but insists it will retain sovereignty.

Warnings on both sides

The Polisario-declared Sahrawi Arab Democratic Republic (SADR) in a statement Wednesday warned: "all states to abstain from any activity on Sahrawi national territory, which is in a situation of open war".

"Our people are being subjected to war and therefore reserve the legitimate right to self-defense in accordance with international law," it said.

The SADR "however is open to all African and international efforts for a peaceful settlement to the conflict," it added.

The United Nations said exchanges of gunfire erupted between Morocco and the Polisario overnight Tuesday-Wednesday, without giving details of any casualties.

The Polisario said its fighters had "inflicted human and material losses" in attacks along the front Tuesday, but there was no independent confirmation.

Morocco's king warned Monday that his country would react with the "greatest severity" to any attack in the territory, as the Polisario said the conflict would continue until Rabat ended its "occupation" there.

King Mohammed VI, speaking after a telephone call with UN Secretary-General Antonio Guterres, said Morocco remained committed to a ceasefire.

But Morocco also "remains firmly determined to react, with the greatest severity, and in self-defense, against any threat to its security", he said, quoted in an official statement.

Morocco has not announced any withdrawal of its troops from the territory.