Clashes between Philippines troops and Abu Sayyaf militants on a southern island killed 23 people Saturday, an army spokesman reportedly said Sunday. The number of dead included 18 soldiers and five Muslim rebels linked to the Islamic State group.

The assault, which continued for ten-hour after the military attacked a stronghold of Abu Sayyaf rebels on the island of Basilan, also injured more than 50 soldiers. The militants were led by Isnilon Hapilon, an insurgent for whom the U.S. State Department has offered a bounty of up to $5 million, Major Filemon Tan said, according to Reuters.

"I can confirm 18 soldiers were killed and 53 were wounded," Tan said, adding that the five militants, who died in the clashes, included a Moroccan, Mohammad Khattab, and Ubaida, a son of Hapilon.

A regional military spokesman told Agence France-Presse that at least four of the soldiers were beheaded during the clashes with more than 100 Abu Sayyaf militants.

"Our group was heading to attack them. On the way, they were ambushed," Col. Benedict Manquiquis, spokesman for the army unit involved in the battle, told radio station DZRH. "The enemy had the high ground so no matter where our soldiers fled to seek cover, they could still be hit by the heavy firepower and improvised explosive devices."

Abu Sayyaf, which is known for extortion, kidnappings, beheadings and bombings, was established in the early 1990s in Basilan province, about 100 miles northeast of Sulu.

On Friday, the group reportedly released a retired Italian priest being held hostage after six months in captivity. Eighteen other foreign hostages, including two Canadians and a Norwegian, are being held in the Philippines, according to reports. The extremist group is reportedly holding the hostages at its stronghold in Jolo island, near Basilan island.