Zamboanga – Philippine soldiers killed four more militants as they pressed their offensive against the Abu Sayyaf after suffering heavy losses in weekend clashes in the troubled south, authorities said on Monday. Troops killed the four on the strife-torn island of Basilan on Sunday, a day after the guerrillas killed 18 soldiers in the worst fighting so far this year, regional military spokesperson Major Filemon Tan said."It is continuous. There will be no let-up in the operations," Tan told reporters, adding that the military was determined to get Abu Sayyaf leader Isnilon Hapilon.Hapilon is one of Abu Sayyaf's most senior leaders. The United States has placed a reward of five million dollars on his head for his role in the kidnapping of three Americans in 2001, two of whom later died.Nine Abu Sayyaf fighters were also killed in Saturday's battle, including Moroccan national Mohammad Khattab, Tan told reporters, bringing the group's total losses to 13.Tan said Khattab was an instructor in bomb-making who was trying to unify the various outlaw groups in the south and establish links with "international" groups."We pre-empted the possibility of bombing attacks. He can no longer teach his terroristic tradecraft," he said, but played down fears the Moroccan could have been linked to the Islamic State group.In Manila, the defence secretary and military chief of staff briefed President Benigno Aquino on the offensive and assured him that "in accordance with his instructions, pursuit operations are still being conducted and that the troops are fully equipped and adequately supported," a presidential spokesperson said.The Abu Sayyaf, a small group of militants notorious for kidnapping foreigners and demanding huge ransoms, was established in the early 1990s with seed money from Osama bin Laden's Al Qaeda network.Based in the southern islands of Basilan and Jolo, it has been blamed for the country's worst terror attacks, including a 2004 Manila Bay ferry bombing that claimed 116 lives.Its leaders have in recent years pledged allegiance to the Islamic State group that controls swathes of Iraq and Syria.The group has stepped up its activity in recent weeks, abducting Indonesian and Malaysian seamen travelling near the Philippines' maritime borders.