Twelve soldiers, including a young lieutenant, were killed in a clash with Islamic State-linked rebels on a remote southern island in the Philippines, an army spokesman said on Monday as the army offensive entered its fifth day.

Major Filemon Tan said five soldiers were also wounded in an 1-1/2 hour firefight in the jungles of Patikul town on Jolo island as troops pursued a large formation of the small but brutal Abu Sayyaf group.

“The fighting was really intense, we lost 12 men,” Tan said. “You can really expect heavy casualty from both sides due to volume of fire from both sides. We don’t know how many from the enemy died but there could be more than 30 rebels.”

A young army lieutenant leading the troops was killed in a rebel ambush.

More than 20 Abu Sayyaf rebels had died since Thursday when the military launched an air-and-ground offensive in Patikul, an Abu Sayyaf stronghold, after President Rodrigo Duterte ordered troops to “destroy” the militant group.

The Abu Sayyaf, known for kidnapping and beheading captives, has dogged successive Philippine governments, entrenching its network with vast sums of ransom money in what has become one of Asia’s most lucrative kidnapping rackets.

Security experts say the Muslim rebels are motivated less by Islamist ideology and more by the tens of millions of dollars from kidnappings.

Two Canadians and a Filipino teenager were executed this year by the Abu Sayyaf. Last week, two Indonesians escaped captivity but there was speculation the Abu Sayyaf freed them after their families paid their ransoms.

Eight Indonesians, eight Filipinos, five Malaysians, a Dutch bird watcher and a Norwegian resort manager are still being held by the Abu Sayyaf, It was not known the hostages were in Patikul jungle when the assault began on Thursday.