An investigation by the United States Department of Defense Pentagon has found that at least 105 civilians died in an anti-Daesh strike in the Iraqi city of Mosul in March, officials said, but they blamed the toll on a secondary explosion of jihadist munitions, AFP reported on Friday.

A U.S aircraft delivered a single precision-guided bomb into a building in west Mosul on March 17, with the aim of killing a pair of snipers on the second story of the structure in the al-Jadida neighborhood, which at the time was under Islamic State control.

But the bomb also caused a large cache of Daesh explosives to detonate, leading to the catastrophic collapse of the building that had civilians sheltering downstairs, officials said Thursday as quoted by AFP.

"The secondary explosion triggered a rapid failure of the structure which killed the two ISIS snipers, 101 civilians sheltered in the bottom floors of the structure and four civilians in the neighboring structure to the west," said U.S Air Force Brigadier General Matt Isler, the lead investigator.

Isler said another 36 civilians who are "believed to be connected" to the building remained unaccounted for, but they had likely fled the area shortly before the strike. He said he was "very confident" in the final toll.

It was the single deadliest incident for civilians stemming from a coalition strike since anti-IS operations in Iraq and Syria began nearly three years ago.

The United States had previously only acknowledged that it "probably" had a role in the civilian deaths.

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

The Pentagon denies this and says its rules of engagement remain unchanged and insists its precision-targeting abilities are the best in the world.

Officials say the US takes every precaution to avoid hitting civilians, including by aborting missile strikes at the last moment if a civilian unexpectedly wanders into the target zone.

"Our condolences go out to all those that were affected," said Major General Joe Martin.

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

No condolence payments have been made, Isler said, though such a move has not been ruled out.