Armed mountain tribesmen have raided an elementary school and nearby homes in the troubled southern Philippines and are holding at least 57 people hostage to keep police at bay, authorities said.

The raid was carried out this morning, less than three weeks after a massacre in a nearby province in which 57 people were killed, throwing an unwelcome spotlight on the Southeast Asian nation and raising tensions ahead of presidential elections next year.

The hostages were being held in a mountainous area near the town of Prosperidad in Agusan del Sur province. Police said the gunmen had seized 75 people, but later freed 18, including all 17 children.

Authorities described the gunmen as former members of a civilian milita who had taken to banditry. Some officials said they could have taken hostages because they were being pursued by police after a gunbattle with a rival tribal group on Wednesday.

Other officials said the group was demanding the dropping of cases against them, action against the rival group and media coverage.

Lino Calingasan, regional police chief, said all remaining hostages were adults.

"Negotiations are ongoing. We are trying to find out how the others can be released," he told Reuters. "It is a good signal, that they are willing to negotiate. We are hoping this will be resolved peacefully."

Negotiators had left the mountain site to return to town and would resume talks on Friday, officials said.

Last month, 57 people, including 30 journalists, were killed after being stopped at a checkpoint in Maguindanao province while on their way to file a candidate's nomination for elections.

CRACKDOWN, MARTIAL LAW

The killings prompted a crackdown in the generally lawless southern Philippines and the imposition of martial law in Maguindanao last week.

Bandits, communist guerrillas and Islamic rebels operate widely throughout Mindanao -- a southern island and region which contains Maguindanao. In addition, powerful local families maintain large private armies and feuding among them is common.

Clan rivalry was at the root of last month's massacre.

The government has armed many villagers and formed civilian militias to counter the communist and Muslim rebels. Analysts say clan wars pose a threat to a fragile four-year truce between the government and Muslim separatists.

Studies funded by the Asia Foundation and the U.S. Agency for International Development found there had been more than 1,200 clan feuds in the south since the 1930s, killing nearly 5,000 people and displacing tens of thousands.

The army dispatched troops to help tackle the situation in Prosperidad, Lieutenant General Raymundo Ferrer said.

"We've sent troops to help rescue the hostages," Ferrer told Reuters. He said the gunmen had been blamed for several robberies and killings in the area.

"We are not aware of any political demands but negotiations are now ongoing to free the hostages that include two forest rangers and some of the parents who were in the school at that time. We're only playing a support role there."

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