Islamic militants linked to Al Qaeda killed seven Philippine soldiers and injured 21 in a five-hour battle Thursday at what appears to be a major jungle base on southern Jolo island.
The marines happened upon a large number of Abu Sayyaf militants as they pursued a smaller group wanted for a series of recent kidnappings, Reuters reported.
Lieutenant-Colonel Randolph Cabangbang said helicopter gunships were called in to support the marines, who appear to have been surprised by the size of the militant force they encountered.
"We're sending helicopters to provide close air support as well as evacuate our casualties," Cabangbang said, adding that the soldiers captured the base and inflicted heavy casualties on the militants.
"Our troops were pursuing a small group of armed men who were holding captives when they found themselves in the middle of the Abu Sayyaf's main base. That would explain our losses."
Abu Sayyaf is the smallest of the Muslim groups fighting the government of the majority Christian Philippines, but it is responsible for the most high-profile attacks including multiple kidnappings and bombings.
Cabangbang said Indonesian and Malaysian militants were thought to have been at the hidden base near Patikul town.
"We hit the militants' nerve center, so there was heavy resistance," he said.
Associated Press said about 30 marines encountered more than 50 rebels in the mountain hideout after maneuvering overnight in search of the kidnapping suspects.
The militant base was commanded by Radulan Sahiron, who is wanted by U.S. and Philippine authorities for a bombings and kidnappings, AP reported.
U.S. Marines provide training and intelligence to their Philippine counterparts in the battle against Abu Sayyaf, which is on a U.S. blacklist of terrorist organizations.
Founded with support from Al Qaeda in the 1990s, it is currently believed to be holding hostages including two Americans, an Indian, a Japanese and a Malaysian.