MARAWI CITY -- Foreign fighters, including Chechens and Arabs, are among the bandits killed in weeklong fighting in this city that involved about 500 gunmen aligned with the Islamic State group, the Defense chief said Thursday, June 1.

An Air Force plane dropping bombs on the bandits killed 11 soldiers and wounded seven others in "friendly fire" as government forces were struggling to rout the assailants in Marawi, the military said.

Defense Secretary Delfin Lorenzana said in a televised news conference that eight of the foreign bandits slain in intense street combat included a Chechen, a Yemeni and several Malaysian and Indonesian fighters.

Lorenzana said that 25 other bandits who died in the fighting had been identified as Filipinos.

There were a total of 120 bandits killed since last Tuesday, May 30, when a failed government raid to capture one of Asia's top terrorist in Marawi triggered the siege of the city by the rebels.

President Rodrigo Duterte declared martial law in Mindanao and poured in troops backed by airstrikes to push the attackers out of the city.

The Marchetti S-211 jet was on a bombing run over the Maute Group's positions in Marawi on Wednesday when one bomb hit Army troops locked in close-range combat with the extremists who have taken cover in buildings and houses, military spokesman Brigadier General Restituto Padilla said.

The plane had made three successful bombing runs before dropping the wayward bomb, he added.

Padilla said the military has ordered an investigation on the friendly fire, which reflects the difficulty faced by government forces to contain the insurrection despite their overwhelming number and firepower.

"Despite the stringent procedure followed, there happens even with the best of armed forces, a case of friendly fire hitting our own troops," Padilla told The Associated Press.

Military chief of staff General Eduardo Ano has ordered an investigation partly to prevent another incident, Padilla said, as government forces, backed by more than 30 assault aircraft, continue to advance toward the remaining positions of the militants.

At least 25 soldiers and five policemen also have been killed in clashes. (AP)