Brigadier General Restituto Padilla told a news conference the soldiers were killed and wounded by a rocket bomb fired by an SF-260 attack plane around noon on Wednesday in an apparent "friendly fire" incident.

"One of our SF-260 aircraft was conducting an airstrike when the last ordnance round it fired went wayward for an unknown reason and accidentally hit and cost the lives of our ground forces," Padilla said.

He added: "This is a case of friendly fire. It was an accident."

Defense Secretary Delfin Lorenzana broke the news of the incident earlier on Thursday.

"Maybe the coordination was not properly done so we hit our own people. We don't know yet what exactly happened," he said.

"Sometimes in the fog of war a lot of things could happen. Accidents happen, like this.

"It's very sad to be hitting our own troops," he added. "There must be a mistake somewhere, either someone directing from the ground, or the pilot."

Lorenzana later said the military is sending more troops to Marawi to fight the remaining 50 to 100 extremists still holed up in the embattled city.

"The situation is very fluid. We're almost there. There is only one pocket of resistance," he said.

A battalion of marine soldiers landed in Marawi City on Thursday to augment the troops on the ground, he said.

The military said ongoing clashes have killed more than 90 militants, 19 civilians and 36 soldiers and policemen.

Lorenzana also said that at least eight foreign terrorists were among the dead militants.

"There were two from Saudi Arabia, two from Malaysia, two from Indonesia, one from Yemen and one from Chechnya," he said.

"We don't have any record of them coming through the proper channel, through the airports. There's only one way, maybe coming from Indonesia or from Malaysia."

On May 23, heavily-armed militants attacked the city of about 200,000 people when the military was searching for a leader of the Abu Sayyaf group, who was hiding in the city.

The troops launched a counteroffensive that resulted in a series of clashes.

President Rodrigo Duterte is concerned radical ideology is spreading in the southern Philippines and it could become a haven for militants forced out of Iraq and Syria.

He made no mention of the killing of soldiers by air force planes in a speech on Thursday. He said the Maute group was being given too much credit, and that the occupation of Marawi was the work of Islamic State, and planned a long time ago.

"You know, the rebellion in Mindanao, it's not Maute, it's purely ISIS," he said.

Xinhua