Hundreds of soldiers launched an assault on two jungle encampments of Al Qaeda-linked militants in the southern Philippines on Wednesday, killing at least 20 gunmen and seizing bombs that had been set to explode, military officials said.

The simultaneous, pre-dawn attacks on two Abu Sayyaf encampments in hilly Silangkum and Baguindan villages on Basilan Island sparked fierce fighting that continued late in the day. The number of troop casualties was not known, Philippine navy Rear Admiral Alex Pama said.

About 500 army troops, marines and police special commandos targeted Abu Sayyaf chieftains Khair Mundus and Furuji Indama, but it was not immediately clear if they were among the dead, said Pama, who helped oversee the assault. Military officials have blamed the two for past bombings and kidnappings.

All the dead were recovered by troops in Baguindan. Soldiers and marines may find more slain militants when they scour Silangkum on Thursday, said regional military commander Maj. Gen. Benjamin Dolorfino, adding troops were pursuing small pockets of fleeing gunmen.

More than 100 troops staged the attacks against about 60 militants in the two strongholds. About 300 others blocked escape routes.

Troops found several bombs, booby traps and 15 assault rifles in Baguindan, Dolorfino said, adding the explosives have been safely detonated by troops.

"The bombs were already primed to explode," Pama told The Associated Press by telephone, adding the explosives may have been intended for another wave of terror attacks.

The Abu Sayyaf, which has about 400 gunmen on Basilan and nearby Jolo Island and the Zamboanga peninsula, is on a U.S. list of terrorist organizations because of bombings, ransom kidnappings and beheadings of hostages. The group is suspected of having received funds and training from Al Qaeda.

The Abu Sayyaf has been weakened by yearslong U.S.-backed offensives, but has turned to kidnappings for ransom in recent months.

Security officials worry that ransom payments could revive the group and have been cracking down on the Abu Sayyaf in Jolo and Basilan — two predominantly Muslim regions that are among the country's poorest.