A US-led air raid in March against a building in the Iraqi city of Mosul killed at least 105 civilians, the Pentagon admitted on Thursday after concluding an investigation into the attack.

According to the Pentagon's summary of the investigation, the March 17 strike on Mosul's al-Jadidah district targeted two snipers of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in the building who had engaged with Iraqi counterterrorism forces.

The Pentagon said, however, that the high civilian death toll was caused by a secondary explosion of munitions placed by ISIL fighters in the building, where civilians were also sheltering.

"There is no way the munition a US plane dropped March 17 in the Jadidah neighbourhood could have caused the extensive damage that killed 101 people in a house and four in a neighbouring house,"it said in a statement.

The report's summary added that the coalition "could not have predicted the presence of civilians in the structure prior to engagement" and that an additional 36 cilivians remain unaccounted for.

Samuel Oakford of Airwars, a London-based collective of journalists and researchers that tracks civilian deaths in Iraq and Syria, told Al Jazeera that "any armed actor in Mosul has to assume there are civilians nearby because that's almost always the case".

READ MORE: Coalition says it hit Mosul site where civilians died

The Mosul strike was the single deadliest incident for civilians arising from a coalition strike since anti-ISIL operations in Iraq and Syria nearly three years ago.

Initial reports from those inside Mosul said that anywhere between 130 to more than 200 civilians died in the strike.

"About 200 people are thought to have died in that strike alone," Al Jazeera's Hoda Abdel-Hamid reported from Erbil in northern Iraq in March.

"These reports of a high toll of civilian casualties were first given by the civilians who actually managed to get out of western Mosul."

No condolence payments

US Central Command (CENTCOM) said in a statement that a local commander had ordered the air raid, which was then carried using a single precision-guided bomb.

It added that it was "the most appropriate and proportionate means of engagement to neutralise the threat and to minimize collateral damage".

Major-General Joe Martin, the commanding general of coalition forces, expressed condolences to those affected.

"The coalition takes every feasible measure to protect civilians from harm. The best way to protect civilians is to defeat ISIS."

US Air Force Brigadier-General Matt Isler, who led the investigation, said no condolence payments have been made, though such a move has not been ruled out.

The investigation comes amid broader claims that US forces under President Donald Trump are killing more civilians as the military fulfils a plan to "annihilate" ISIL.

On Thursday, the Syrian Observatory for Human Rights (SOHR) said US-led coalition air raids killed at least 35 civilians in a town in Syria's Deir Az Zor province.

Earlier this week, SOHR said a total of 225 civilians were killed in US-led coalition air raids between April 23 and May 23 - the highest monthly civilian death toll for the coalition since it began bombing Syria on September 23, 2014.

Airwars has said a minimum of 3,350 people have been killed in coalition strikes in Syria and Iraq.

As of the most recent US military count at the end of April, a total of 396 civilians had been killed since the beginning of the bombing campaign against ISIL nearly three years ago.

The 105 figure from the March incident would push that number beyond 500.