The Moro National Liberation Front (MNLF) has invaded the city in an attempt to derail a rival rebel group's planned peace pact with the government.

More than 100 people held by the rebels were released overnight.

The military says a sustained offensive, which began on Friday and included helicopter rocket attacks, was proving a success.
"For the past 18 hours, we have been able to rescue 116 people due to pressure from our troops on the ground," military spokesman Lieutenant Colonel Ramon Zagala told AFP.

Officials say the death toll has reached 87, with 71 rebels, nine security forces and seven civilians killed.

The mayor of Zamboanga, Isabelle Salazar, says a two-year-old boy, who was among the captives, was among those who died.

"They were given an opportunity to freedom, but in of the canals where the parents hid, he was hit in the head by a bullet."

Government officials have admitted they are finding it difficult to feed the growing number of evacuees.

More than eighty thousand people have been displaced by the fighting.

Security officials say MNLF leader Nur Misuari is on the retreat, and his men have been pushed back to the shoreline of a coastal village by military bombardments.

About 200 rebels initially took dozens of hostages and burned hundreds of homes, forcing a shutdown of Zamboanga, a city of about one million and key commercial hub in the region.

Philippine troops have fired rockets from helicopters in an attempt to dislodge the rebels.

'Precision' strikes

On Monday MNLF rebels torched a section of Santa Barbara, one of the neighbourhoods they had occupied, to slow down the military advance.

Asked about the potential for the civilians to be caught up in the helicopter assaults, Lieutenant Colonel Zagala emphasised they were "precision" strikes.

Human Rights Watch has expressed concern about the growing dangers for civilians, four of whom had been killed in earlier stages of the conflict.

"This definitely raises the level of danger for civilians in the area," said Carlos Conde, the Philippine representative for the New York-based watchdog.

"These are residential areas, how would they (soldiers) know which house or area to target? I don't think they would have that precise information."

Schools in Zamboanga have closed and ferry and air services suspended.

Muslim rebels have been fighting since the 1970s for an independent or autonomous homeland in the south of the mainly Catholic Philippines. An estimated 150,000 people have died in the fighting.

The MNLF signed a peace treaty in 1996 that granted limited self-rule to the south's Muslim minority, and has since largely participated in the country's political process rather than foment violence.

But MNLF founder Nur Misauri has been angered by a planned peace deal between the government and rival group the Moro Islamic Liberation Front, as he believes it would sideline his organisation.