SHARIFF AGUAK - Philippine bomber planes staged a show of force Tuesday over a southern province placed under martial law, as helicopters dropped leaflets urging a warlord's militia to surrender.

The military flights over Maguindanao province were aimed at intimidating the 3,000 gunmen loyal to a Muslim clan chief whose family is accused of being behind last month's political massacre of 57 people.

"We want to show to this group, which does not want to be disarmed, our capability if we decide to launch a tactical offensive against them," Col. Leo Ferrer, an Army ground commander, told reporters.

As the two OV-10 Bronco bomber planes flew over remote hilly areas where the militiamen are believed to be entrenched, helicopter gunships dropped leaflets urging them to end their rebellion.

The leaflets contained messages in the local dialect calling on the militiamen to "surrender for the sake of peace in Maguindanao", Ferrer said in the provincial capital, Shariff Aguak.

President Gloria Arroyo placed Maguindanao under martial law on Friday night, then deployed thousands of troops to crush the Ampatuan clan and its forces.

Ampatuan family leaders are accused of masterminding the massacre of 57 people on November 23, a crime allegedly meant to stop a member from a rival clan from pursuing the Maguindanao governorship in elections next year.

The clan chief, Andal Ampatuan Sr., had been governor of the province and an ally of Arroyo's government since 2001.

But his son, Andal Ampatuan Jr., was accused of leading 100 gunmen in abducting, then shooting dead, female relatives of the rival clan member, plus a group of journalists.

Ampatuan Jr. was charged with 25 counts of murder shortly after the massacre.

Ampatuan Sr. and four other relatives were among 62 people arrested in raids after martial law was implemented. Authorities have said the patriarch will be charged with rebellion and possibly murder.

Raids on Ampatuan properties have led to the seizure of huge caches of high-powered weapons, and the government has said the clan intended to use them in their fight to hold on to power in Maguindanao.

Police said they had uncovered another large haul of grenade launchers and other weapons on Tuesday at a compound where Zaldy Ampatuan, another son of the patriarch who is also facing rebellion charges, worked.

Troops have been placed on the highest state of alert, after the militiamen shot at patrolling police commandos Sunday, triggering a gun-battle but no casualties.

"The threat is still there," military spokesman Lieutenant Colonel Romeo Brawner said in Manila. "Our troops are on high alert in the area against any further attacks. We are doubling our efforts."

But he also said that forces on the ground had received surrender overtures from relatives of the militiamen.

"They are in the outskirts, in remote areas in Maguindanao (and) running low on food," Brawner said.

The Ampatuans were one of several clans supported by Arroyo's government as part of a strategy to contain a Muslim insurgency that has raged across Maguindanao and other parts of Mindanao island since the 1970s.

The insurgency has claimed at least 150,000 lives, the military says.

The government allowed the Ampatuans and other clans to build up their own security forces to help overstretched military and police units on Mindanao.

Amnesty International on Tuesday raised concern about the martial law arrests, but said it had not monitored any serious violations by troops.

Nonetheless, the rights group asked the government to establish "a clear and short timetable" to lift martial law.