AFP/Manila

Philippine security forces and a rebel group said on Sunday they had clashed in the country's south with an unspecified number of casualties, in violence rarely seen since the signing of a peace treaty.

Police commandos and Moro Islamic Liberation Front (MILF) members fought near the remote town of Mamasapano on the main southern island of Mindanao before dawn, regional military spokesman Captain Joan Petinglay said.

"Ceasefire monitors are now on the ground to separate the forces and remove the casualties," said Petinglay, speaking to AFP by telephone from the neighbouring town of Shariff Aguak.

Mohagher Iqbal, the lead MILF negotiator in a landmark peace deal signed in March last year, also confirmed the incident, but neither side would say how many fighters were wounded or killed, or to whom they belonged.

The 10,000-member MILF had agreed to end decades of rebellion in the mainly Catholic nation in exchange for a proposed law now being debated in parliament that would give the minorities self-rule in several southern provinces.

"This is the first encounter between the MILF and (government forces) this year. Hopefully, this will be the last," Iqbal told AFP by telephone.

He said the police had entered an MILF-influenced area without notifying the group first, while searching for members of the Bangsamoro Islamic Freedom Fighters (BIFF), which disagrees with the peace talks and broke away from the MILF in 2008.

"They (police) ran into an MILF force. The ceasefire monitors are now in the area," Iqbal added.

"We heard some people were killed, but I believe this will not affect the peace process," he added.

Both Iqbal and Petinglay said a joint government-MILF ceasefire committee and a small international monitoring team of soldiers and police from Malaysia and other countries had arrived in the area by the afternoon to "disengage" the two groups.

The clash in Mamasapano, about 900 kilometres (559 miles) south of Manila, was only the second since two soldiers and 18 gunmen were killed in a clash on the southern island of Basilan in April 2014.

Such incidents once broke out with much greater frequency prior to the signing of the treaty, during a rebellion that has claimed tens of thousands of lives.

After the April fighting, the government accused the MILF of helping Islamic extremists under attack from security forces. The group acknowledged four of its members were killed.

Since the peace deal was struck, government forces have been going after the BIFF, a group of several hundred gunmen who last year pledged allegiance to Islamic State fighters in Iraq and Syria.