At least 15 Philippine soldiers were killed in a clash between the army and Abu Syyaf militants, authorities said on Tuesday.The clash took place on Monday afternoon in Patikut in Sulu province when a military patrol was attacked by about 120 heavily armed militants, the chief of the Public Affairs Office of the Armed Forces, Edgard Arevalo, said.He said the number of militants killed in the shooting was still unknown, reported EFE news.Abu Sayyaf beheaded two Canadian hostages, John Ridsdel and Robert Hall, in April and June, respectively, after deadlines for ransom payment were not met.The group is presently holding a Norwegian, a Japanese, nine Indonesians and five Malaysians.