The marines were killed when their unit attacked a jungle hide-out of the Al Qaeda-linked Abu Sayyaf group near Basilan's remote town of Ungkaya Pukan, sparking a daylong clash during which an air force pilot also died when his helicopter crashed into the sea, the military said.

The military claimed about 40 Abu Sayyaf insurgents were killed, including two commanders who allegedly took part in last month's beheadings of 10 marines on Basilan.

Lt. Gen. Eugenio Cedo, a regional military commander, said the marines apparently underestimated the rebels' firepower.

"They were not able to assess properly the strength of the enemy during the assault," Cedo told ABS-CBN television.

Saturday's clash involved about 80 Abu Sayyaf gunmen, Cedo said, adding that hundreds of troops were pursuing those who survived. The bodies of slain rebels were scattered around the battle scene, he said.

Military chief of staff Gen. Hermogenes Esperon said Isnilon Hapilon, an Abu Sayyaf commander wanted by the U.S. and Philippine governments for alleged kidnappings and bomb attacks, was among those being hunted by troops in Basilan. It was not clear whether Hapilon was involved in Saturday's fighting.

"We are making a promise to make the province unfit for those who want to rule it with arms," Esperon said.

Troops fired 105 mm howitzer shells early Sunday at suspected Abu Sayyaf positions near Ungkaya Pukan and the nearby townships of Sumisip and Tipo Tipo but there was no immediate report of casualties, officials said.

On nearby Jolo island, troops raided a suspected Abu Sayyaf safe house in Indanan township early Sunday. They took into custody 19 men, women and children and seized four M-16 rifles and ammunition, army Maj. Gen. Ruben Rafael said.

After questioning, 14 were released but five men remained in military custody while investigators tried to determine whether they were Abu Sayyaf gunmen, Rafael said.

Military spokesman Lt. Col. Bartolome Bacarro said U.S. forces involved in counterterrorism training in the country's south were providing intelligence to help Philippine troops track down the Abu Sayyaf militants. The Americans have also provided combat training to local troops.

President Gloria Macapagal Arroyo had ordered a fresh offensive against the Abu Sayyaf, a small but violent group listed by Washington as a terrorist organization, after it was implicated in the July 10 beheadings of 10 marines following a clash in Basilan's Al-Barka township.

A larger Muslim rebel group engaged in peace talks with the government, the Moro Islamic Liberation Front, admitted its rebels killed 14 marines during the clash in Al-Barka, accusing them of encroaching into a rebel stronghold.

But the MILF denied its men beheaded 10 of the 14 marines.

Angered by the beheadings, the 120,000-strong military launched a new offensive against the Abu Sayyaf on Basilan and Jolo island.

Nearly 24,000 people have been affected or displaced by the fighting in Basilan and Jolo.