The pro-independence Polisario Front on Saturday reported clashes in the Western Sahara after Morocco launched an operation in the buffer zone in the disputed territory, as the UN led calls for restraint.
Tensions in the former Spanish colony have sparked concern around the globe, with the United Nations, the African Union, Algeria and Mauritania urging both sides to respect a 1991 ceasefire.
But senior Polisario official Mohamed Salem Ould Salek said on Saturday that the ceasefire supervised by the United Nations is a "thing of the past".
"Fighting is continuing after the crime committed by Moroccan troops in Guerguerat," Ould Salek, a foreign minister of the Polisario-declared Sahrawi Arab Democratic Republic (SADR), told AFP.
SADR President and Polisario Secretary-General Brahim Ghali said he had "declared war" on Friday and announced "the resumption of armed actions in order to protect the inalienable rights of our people".
On Friday, Morocco announced that its troops launched a military operation in the buffer zone of Guerguerat in order to reopen the highway to Mauritania, accusing the Polisario of blocking the road.
Dozens of truck drivers have been stranded for days at Guerguerat, the last Moroccan-held stop before the road enters the buffer zone along the border where the Polisario has kept a periodic presence.
The road is key for trade with the rest of Africa.
Late Friday the SADR defence ministry said Sahrawi forces had carried out "massive attacks" during the afternoon at multiple points along Morocco's 2,700 kilometre (1,700 mile) long defensive wall.
It said they had inflicted "human and material damage on the enemy".
The claim was denied in Rabat by a "well informed source" which said that Morocco's army chief of staff and the UN's MINURSO mission did not report any attacks.
Morocco's military, later Friday, said it had secured the Guerguerat crossing between Morocco and Mauritania by installing "a security cordon" along its desert wall.
Calls for restraint
The unrest in Western Sahara has sparked concern around the world.
UN Secretary-General Antonio Guterres has expressed "grave concern regarding the consequences of the latest developments" that erupted despite UN efforts to avoid an escalation.
Stephane Dujarric, spokesman for the UN chief, said that MINURSO deployed in Guerguerat "a special civil-military team... since the beginning of the crisis" and military observers were there overnight.
Algeria, which back the Polisario, and Mauritania have called for restraint and respect of the 1991 ceasefire.
They were echoed by the African Union, France, Russia and Spain.
AU Commission chairman Moussa Faki Mahamat on Saturday expressed his "deep concern following the deterioration of the situation... and the serious threats of breaching the ceasefire".
Western Sahara's former colonial power Spain called on "the parties to resume the negotiation process and move towards a political, fair, lasting and mutually acceptable solution."
France called on all sides to "avoid an escalation" and Russia urged them "to refrain from taking steps that can exacerbate the situation".
Western Sahara, a vast swathe of desert on Africa's Atlantic coast, is a disputed former Spanish colony. Rabat controls 80 percent of the territory, including its phosphate deposits and its lucrative ocean fisheries.
Morocco, which maintains that Western Sahara is an integral part of the kingdom, has offered autonomy for the territory but insists it will retain sovereignty.
The Polisario demands a referendum on self-determination as set out in the 1991 ceasefire.
The vote has been repeatedly postponed due to disputes between Rabat and the Polisario over voter rolls and the question to be asked.