The Philippine armed forces have launched a helicopter airstrike as part of their ongoing assault against Muslim rebels occupying parts of a major southern city, stepping up efforts to end an eight-day standoff that has left dozens dead.

Two air force helicopters fired rocket rounds toward Moro National Liberation Front (MNLF) rebels held up in coastal villages in Zamboanga city on Monday, the military said.

"This is a precision close air support directed by ground troops to suppress the enemy," military spokesman Lt. Col. Ramon Zagala told AFP.

The helicopter assault was the first air strikes since troops began their offensive on Friday to defeat the MNLF forces, who have been using civilians as human shields.

Zagala said earlier on Monday up to 100 MNLF rebels were still engaged in ground battles with troops around two coastal villages, a week after the guerrillas invaded Zamboanga to stake an independence claim.

Zagala said the rebels were defiant in the face of the military advance.

"They still have ammunition, and they continue to fire at us," Zagala said from Zamboanga, but insisted the military offensive was succeeding.

"We know for a fact that the end is near, and they are trying to flee. Some of them may be trying to disguise as civilians, so it's very critical that the village elders help us identify those who are not from their neighborhoods."

The latest violence in Zamboanga has left 51 MNLF fighters, six security forces and four civilians dead, according to Zagala.