(Corrects president's name in paragraph 7 to "Aquino" in story released on Dec 15, 2015)
MANILA (Reuters) - Philippine troops clashed on Tuesday with Islamist militants on a remote southern island and 15 people were killed as security forces step up an offensive against rebels who beheaded a Malaysian captive, an army spokesman said.
Militants from the Abu Sayyaf group had steered clear of government troops on the islands of Jolo and Basilan since the beheading of a Malaysian restaurant owner last month.
The al Qaeda-linked Abu Sayyaf, known for kidnappings, beheadings and bombings, are one of the most brutal Muslim rebel factions in the Muslim south of the largely Christian Philippines.
Army troops killed 13 rebels in the gun battle that erupted at 5 p.m. (0900 GMT) in the jungles near the town of al Barka on Basilan island, army spokesman Filemon Tan said. Two soldiers were also killed and 10 were wounded, he added.
Although soldiers had been in the area since Sunday, Tan said, "It was only today when the fighting got bigger."
Sporadic skirmishes were continuing, he said.
The troopers found a major rebel base on the island, Tan said, adding that the military had launched an offensive against the rebels since Nov. 18, when President Benigno Aquino ordered them to hunt down Abu Sayyaf over the execution.
There was no immediate statement from the rebel group.
Soldiers in nearby Jolo found a body believed to be that of the beheaded Malaysian businessman in a shallow grave on Monday night and took it to a police laboratory for DNA tests, Brigadier-General Alan Arrojado told reporters.
The military spokesman in Manila, Colonel Restituto Padilla,
said an informant had led troops to the site, and added, "The head was sent to the police for DNA testing and now the body for DNA matching."
A clear identification would be necessary prior to notifying Malaysian authorities and the victim's family, he said.
Attending the Asia Pacific Economic Conference in Manila last month, Malaysian Prime Minister Najib Razak had condemned the beheading of the businessman, Bernard Then Ted Fen, calling it a "savage and barbaric act".
The government signed a peace deal with the largest Muslim rebel group, Moro Islamic Liberation Front, in March, promising to grant autonomy in the south and ending a 45-year conflict that killed 120,000 people and displaced 2 million.
Reporting by Manuel Mogato; Editing by Clarence Fernandez