Manila - The Philippine government on Tuesday ordered an investigation into the killings of six civilians during an air assault on Muslim separatist rebels in a southern province.

The civilians, including children and a pregnant woman, were on their way to an evacuation area when their motorboat was hit Monday by rockets fired from a military helicopter gunship.

They were fleeing Datu Piang town in Maguindanao province, 960 kilometres south of Manila, where fighting erupted between the military and the Moro Islamic Liberation Front (MILF), the largest Muslim separatist rebel group in the Philippines.

Press Secretary Jesus Dureza said an investigation was needed to determine if the military followed operational guidelines for offensives during the holy month of Ramadan.

"We need to investigate this incident," he said. "The fact remains that civilians were killed. We laid down operational guidelines not to use bombs and artillery during Ramadan."

Dureza said government forces were only allowed to use heavy weapons "at extreme need" during Ramadan.

Major Gerardo Zamudio, an air force spokesman, said the OV-10 helicopter attacked MILF rebel positions near a marshland in Datu Piang after taking "hostile fire" from the ground.

"The OV-10 was only forced to fire because they could clearly see MILF rebels firing at them," he said. "There were 10 boats being used by the MILF rebels to flee."

Zamudio welcomed an investigation into the incident, saying it would give troops a chance to air their side.

"Under the rules of engagement, if you are under constant enemy fire, you have to engage," he said. "Ground troops said the fire was coming from MILF rebels."

The Philippine Commission on Human Rights and the International Committee of the Red Cross have expressed alarm over the increasing number of civilian casualties in the fighting in the south.

On the weekend, Dominik Stillhart, Red Cross deputy director for operations, urged the military and the MILF to spare civilians in the escalating violence.

About 200 people, including at least 60 civilians, have been killed and more than 500,000 displaced in the fighting that flared up last month. Stillhart said the fighting was the worst since 2003.

The hostilities erupted after the Supreme Court stopped the signing of an ancestral domain agreement that would have expanded an existing autonomous Muslim region on the southern island of Mindanao. After the ruling, the MILF began seizing villages and attacking civilians.

The Philippine government has scrapped the deal because of the hostilities and strong opposition from Catholic officials. It has also dissolved its team seeking to negotiate a peace pact with the MILF. (dpa)