Morocco's king warned Monday that his country would react with the "greatest severity" to any attack in Western Sahara, as the pro-independence Polisario Front said conflict would continue until Rabat ended its "occupation" of the disputed territory.
The United Nations said that both sides had exchanged fire, and urged restraint.
Moroccan King Mohammed VI, speaking after a telephone call with UN Secretary-General Antonio Guterres, said Rabat remained committed to a ceasefire.
But Morocco also "remains firmly determined to react, with the greatest severity, and in self-defence, against any threat to its security", the king said, quoted in an official statement.
The crisis erupted after Morocco launched a military operation Friday to reopen a key highway at the Guerguerat border crossing between the territory 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.
"The end of the war is now linked to the end of the illegal occupation of parts of the territory of the Sahrawi Republic," senior Polisario official Mohamed Salem Ould Salek said on Monday.
"The war only started as a consequence of Morocco's aggression and action in Guerguerat," said Ould Salek, who is foreign minister of the Polisario-declared Sahrawi Arab Democratic Republic (SADR).
He also downplayed the importance of the highway, where trucks had been blocked for weeks. Traffic resumed on Saturday between Mauritania and Morocco, the two countries have said.
"It is not an international or even regional road. It is being used to loot the natural resources of the Sahrawi people," Ould Salek said, accusing Morocco of having started the latest conflict.
Rabat controls around three quarters of the Western Sahara, a vast swathe of desert on the Atlantic coast, including its phosphate deposits and its lucrative ocean fisheries. The Polisario controls the remainder.
Morocco maintains that Western Sahara is an integral part of the kingdom and has offered autonomy for the disputed territory, but insists it will retain sovereignty.
- Exchange of fire -
Ould Salek said a full implementation of the 1991 ceasefire -- namely organising the self-determination referendum set out in the truce deal -- -- was a condition for an end to hostilities.
The vote has been repeatedly postponed due to disputes between Rabat and the Polisario over voter rolls and the question to be put on the ballot.
The Moroccan official news agency MAP said late Sunday that Rabat's military had responded to fire by the Polisario Front along a UN-patrolled buffer zone.
"Since 13 November 2020, Polisario militias have fired provocative shots along the line of defence without causing human or material damage," MAP said, citing the Far-Maroc unofficial website dedicated to military news.
Retaliatory fire from the Moroccans destroyed an armoured vehicle east of the line of defence at El Mahbes, it said on its Facebook page.
The UN on Monday confirmed an exchange of fire.
The MINURSO mission "received reports by both sides of incidents of shooting" overnight Sunday to Monday "at various locations" in the territory, Stephane Dujarric, spokesman for the UN chief, told reporters in New York.
MINURSO, he said, calls on both sides to "exercise restraint" and take measures to defuse tensions.
On Sunday, the Polisario reported intense fighting along the 2,700-kilometre (1,700-mile) Moroccan wall of defence that cuts through Western Sahara.
It also announced that it was mobilising "thousands of volunteers" to join Polisario Front fighters.
The territory is tough to travel through and Moroccan authorities do not allow journalists access, making it difficult to verify reports from either side.
Domestic flights are also suspended because of the coronavirus pandemic.
Negotiations involving Morocco, Polisario, Algeria and Mauritania have been at a standstill since 2019.