At least 11 Philippine soldiers were killed by “friendly” fire in a government air raid during efforts to take back a southern city occupied by fighters, Secretary of National Defense Delfin Lorenzana, said, promising an investigation into the incident.

Seven other soldiers were wounded on Wednesday when two air force SF-260 close air support planes dropped bombs on a target in the heart of Marawi City, Lorenzana told reporters on Thursday. The first plane hit the target but the second missed.

“It’s very sad to be hitting our own troops,” Lorenzana said. “There must be a mistake somewhere, either someone directing from the ground or the pilot.”

Later he told reporters, “We will come up with a report.”

In a separate press briefing, military spokesman Brig Gen Restituto Padilla said the armed forces would form a board of inquiry to investigate whether the incident was caused by an equipment failure or human error.

The armed forces of the Philippines have been using a combination of ground operations by soldiers and helicopter air raids to try to dislodge Maute rebels linked to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, who have occupied parts of Marawi for eight days.

The Maute group has proven to be a fierce enemy, clinging on to the heart of Marawi through days of air attacks the military has said are “surgical” and on known rebel targets.

The deaths of the soldiers take the number of security forces killed to 38, with 19 civilians and 120 rebel fighters killed in the battles in Marawi over the past nine days.

Tens of thousands of people have fled the fighting.

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Lorenzana said Saudi, Malaysian, Indonesian, Yemeni and Chechen fighters were among eight foreigners killed fighting with the Maute rebels.

In an earlier text message to reporters, he said of the “friendly fire” incident: “Sometimes that happens. Sometimes the fog of war … The coordination was not properly done so we hit our own people.”

Thousands of civilians have fled the fighting in Marawi [Romeo Ranoco/Reuters]

‘Relentless air strikes’

Marawi, a mostly Muslim-populated city of 200,000 people, lies about 800km south of the capital, Manila.

President Rodrigo Duterte declared martial law in the south throughout mid-July after fighters went on a deadly rampage in Marawi last week, following an unsuccessful military raid to capture Ismilon Hapilon – a veteran Filipino fighter regarded as the local ISIL leader.

READ MORE: Philippines – ISIL-linked group using children as fighters

“Since the Philippine government announced martial law, there have been relentless air strikes, ‘surgical air strikes,’ as the Philippine military described it,” Al Jazeera’s Jamela Alindogan, reporting from the outskirts of Marawi, said.

“There have been organisations and civilians here who have been asking the government to stop the air strikes, simply because of the danger they pose for civilians.”

More than 2,000 people are estimated to be trapped in the conflict zones in Marawi, fearing for their lives amid violence by fighters and military air raids.