The Pentagon denied Wednesday that large numbers of civilians were killed in a March strike by a Syrian mosque, acknowledging only one possible civilian death at an adjoining religious compound.

The Britain-based Syrian Observatory for Human Rights has claimed that the March 16 strike in the village of opposition-held Al-Jineh in northern Aleppo killed 49 people -- most of them civilians.

But an investigation led by Brigadier General Paul Bontrager found that about two dozen men attending an Al-Qaeda meeting were killed in the strike, with several others wounded.

"Sadly, we did assess that there was likely one civilian casualty," Bontrager said, noting he was "unsure" whether the person survived.

The general said the probe looked at media reports that indicated a large number of civilian deaths, but investigators did not uncover evidence to support those claims.

"We are not aware of large numbers of civilians being treated in hospitals after the strike," he said.

"We are confident this was a meeting of Al-Qaeda members and leaders."

The strike, conducted by F-15 warplanes and Reaper drones, completely destroyed a building next to a small mosque, which investigators now recognize was a madrassa -- or Islamic religious school -- that was still under construction.

The review found that the strike's targeting officers were not made aware that the complex under construction had a general religious purpose, Bontrager said.

"The failure to identify the religious nature (of the building) is a preventable error," he said, adding, "there should have been more questions asked."

While no one was found to be negligent in approving the strike, Bontrager said officers should have better interrogated each other about whether they should hit the structure.

"Any madrassa or any other structure at all, if it's being used for military purpose, can be struck, can be a legal target to strike," he said, "it simply has to go through a different legal authority."

The investigation also found an "overarching problem" of incomplete information flow in the "target engagement" cell, Bontrager said.

"Though the investigation identified some critical information gap that contributed to misinformation and an overall lack of understanding of the situation, we ultimately struck a blow against Al-Qaeda," he said.

Syria's conflict erupted in March 2011 with widespread protests against the regime, but it has since morphed into a multi-front war that has left 320,000 people dead.

The US military has said coalition air strikes in Iraq and Syria had "unintentionally" killed 484 civilians, but observers say the number is far higher.