MANILA — Islamic separatists attacked several towns and villages on Monday in the troubled southern Philippine region of Mindanao in a rampage that killed at least 28 people, mostly civilians, officials said.

President Gloria Macapagal-Arroyo ordered the military and the police “to defend every inch of Philippine territory” against the Moro Islamic Liberation Front, the main Islamic separatist group operating in Mindanao.

The Moro Islamic Liberation Front has fought for an Islamic state in Mindanao for several decades. It has also been involved in tenuous peace negotiations with the Philippine government for more than a decade.

The attacks came as tens of thousands of villagers in other areas of Mindanao were returning to their homes after battles last week between government troops and the Muslim rebels.

Media reports from Mindanao said several victims had been hacked to death with machetes. According to officials, the attackers also burned down houses. The police said that most of those killed were civilians, mainly farmers, though an undetermined number were soldiers.

Officials said that more than 200 rebels had attacked at least four towns in two provinces in Mindanao.

The civilians were killed as the rebels were withdrawing, said Brig. Gen. Hilario Atendido, a military commander in the area. “They used them as human shields,” General Atendido said, speaking on the radio station DZBB. “The rebels killed them on their way out.”

According to news reports, the rebels also took several residents hostage.

Mohammad Khalid Dimaporo, the governor of Lanao del Norte Province, said the rebels were moving toward Christian-dominated towns in the coastal areas of the province. “The military is doubling its forces,” he told ABS-CBN television. “The highest priority now is to secure the coastal towns.”

Eid Kabalu, a spokesman for the Moro Islamic Liberation Front, said that his group had not issued any orders to carry out assaults and that it was still checking into reports about the attacks. He urged the public “not to jump to conclusions.”

The violence this week, which began Sunday in Mindanao when four soldiers and four military-supported militiamen were killed, is certain to complicate peace negotiations between the government and the rebel front.

The army chief, Gen. Alexander Yano, said that the violence on Monday was a “clear manifestation of the insincerity to the peace process of a significant portion,” and that this was “a virtual declaration of war against the duly constituted authority.”

Copyright 2008 The New York Times Company