MANILA, June 1 (Xinhua) -- In a case of apparent "friendly fire," Philippine military airstrikes aimed at Islamist militants "Maute" in the southern city of Marawi killed 11 soldiers and wounded seven others, defense and military officials said Thursday.

Brig. Gen. 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.

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

"This is a case of friendly fire. It was an accident," Padilla added.

Defense Secretary Delfin Lorenzana broke the news about the tragedy earlier Thursday.

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

Lorenzana told another news conference later in the Malacanang presidential palace that an investigation is underway to look into the incident, noting President Rodrigo Duterte has already been informed of the incident.

He 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.

"We have just delivered 21 armored vehicles to Marawi. All of them will be used against the Maute ISIS," Lorenzana 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 fighting along with the Maute IS group 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," Lorenzana said.

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

The troops launched a counter-offensive that resulted in a series of clashes.