Philippine soldiers load bodies of dead Abu Sayyaf Group suspects onto a truck in Jolo, in the southern province of Sulu, Dec. 28, 2016.

Government forces killed at least 20 Abu Sayyaf suspects in three days of clashes in the southern Philippines amid heightened security operations, military officials said, after the U.S. embassy warned that the militants were preparing a fresh wave of kidnappings.

The suspects were killed during a counter-militant operation that began Thursday in Sumisip, a town in Jolo province, the commander of the military’s Joint Task Force in Basilan province said Monday.

Task force chief Col. Juvymax Uy said soldiers under the 4th Special Forces Battalion and 3rd Scout Ranger Regiment of the Philippine Army raided the terror group’s hinterland hideout in the area.

“We launched air strikes and indirect fire towards their position,” he said, adding that residents corroborated a military report that the Abu Sayyaf Group (ASG) had sustained at least 20 fatalities.

The military, however, has not been able to penetrate the rebel-held areas, but an initial sweep of its periphery led to the recovery of three home-made bombs and enough materials and components to make 30 more explosives, he said.

Witnesses have reported that ASG militants left their dead in shallow graves, in keeping with local religious customs, Uy said.

Earlier, the U.S. Embassy in Manila had issued a travel advisory, warning its citizens to stay away from Palawan in the southwest of the country, saying it had received information that the militants were preparing a fresh wave of kidnappings.

Last month, American officials warned their citizens to avoid Bohol province in the central Philippines, a day before a group of Abu Sayyaf gunmen landed in the area.

A running gunbattle ensued for the next week during which the military killed at least nine militants, including an Abu Sayyaf sub-commander blamed for past beheadings of foreign hostages.

The Abu Sayyaf, founded in the early 1990s by Islamic firebrand cleric Abdurajak Abubakar Janjalani, was set up originally to wage war for an independent Islamic state in the southern Philippines. But it has since degenerated into a criminal organization specializing in bombings and kidnappings.

Janjalani and the original top Abu Sayyaf lieutenant have been killed one after the other in government offensives over the past two decades.

One of its original commanders, Radullan Sahiron, is believed to be ailing and has sent surrender feelers to the military. He has a $1 million-reward on his head offered by the U.S. government.

But another faction led by Isnilon Hapilon, who like Sahiron is among the original leaders of the group, has pledged allegiance to Islamic State and has carried out attacks in the name of the Middle East-based group’s black flag.