Ten soldiers have been killed in a government air strike in the embattled city of Marawi in the Philippines.
Eight soldiers were also wounded in the "friendly fire" incident on Wednesday, said defence secretary Delfin Lorenzana, expressing remorse.
Over the last week militants allied to the so-called Islamic State have engaged in street battles with the army leaving more than 100 people dead.
It was sparked by the attempted capture of a top militant leader by the army.
"Yesterday we had a tragedy that involved our troops. A group of our military, army men, were hit by our own airstrike. We lost men, 10 killed and 8 wounded," Mr Lorenzana told reporters.
President Rodrigo Duterte declared martial law on southern Mindanao island, where Marawi City is located, last week.
Security forces launched a raid to arrest Isnilon Hapilon - a Filipino militant on the US's list of most-wanted terror suspects.
Authorities said dozens of fighters fought back to protect him, and then attacked parts of the city with a population of 200,000, taking hostages.
The Philippines military has since been using armed forces and helicopter airstrikes to try and drive the militants out.
Most of the civilians have left Marawi, while about 2,000 remain trapped.
Defence Minister Lorenzana told reporters on Thursday that among the militants that have been killed there were fighters from at least five other nationalities including Saudi, Yemeni and Chechen, according to Reuters.