Military authorities in the Philippines say four crew members of a tugboat towing an Indonesian barge have been kidnapped near a stronghold of the Abu Sayyaf rebel group. The abduction comes 10 days after a shootout in which the rebels lost three hostages.

Philippine authorities say the gunmen boarded the barge from a speedboat, and abducted the Indonesian captain and three crew members Monday night. The barge was found Tuesday drifting near the island of Basilan, 900 kilometers south of Manila.

The barge was carrying freight from Indonesia to the central Philippines port of Cebu.

News reports quote an official with the company that hired the barge as saying the kidnappers were Abu Sayyaf guerrillas. The group advocates a separate Islamic state in the southern Philippines but is mostly known for kidnapping people for ransom.

However, the deputy commander of the Philippine army's Southern Command, Colonel Franscisco Gudani, says it is premature to say the kidnappers are Abu Sayyaf. "Our report did not mention who were the perpetrators," he said. "We only knew of four Indonesians who were taken away by whom we do not know yet. There was also a remaining crew aboard the boat, which was seen drifting toward Basilan. There were no details really because we have yet to interrogate the remaining crew."

The Philippine military is pursuing the Abu Sayyaf, who during the past year kidnapped more than 100 people, mostly civilians. Most of the victims were either ransomed or killed.

The guerrillas lost their last three hostages 10 days ago in a battle with Philippine troops. American missionary Martin Burnham and Philippine nurse Ediborah Yap were killed in the fight. Mr. Burnham's wife, Gracia, was wounded, but was rescued and has returned to her family in the United States.

Philippine officials have warned that the Abu Sayyaf rebels are likely to seek new hostages to use as a shield against Philippine troops. The military has intensified its operations against them in recent weeks.

The Abu Sayyaf also is suspected in an attack Monday night on Philippine and U.S. troops on Basilan island. Unidentified gunmen opened fired on soldiers at a U.S. Navy construction project.

It was the first attack against the American troops, who arrived five months ago to train and equip the Philippine special forces pursuing the Abu Sayyaf. The U.S. military presence in the southern Philippines has been called the second-largest front in the U.S. war on terrorism, after Afghanistan.