U.S.-led coalition forces have killed 173 civilians since starting their 2014 operations against the Islamic State group, also called ISIS, in Iraq and Syria. The Combined Joint Task Force Operation Inherent Resolve (CJTF-OIR) said Thursday 54 civilians died in airstrikes that took place between March 31 and October 22.

“We regret the unintentional loss of civilian lives resulting from Coalition efforts to defeat ISIL in Iraq and Syria and express our deepest sympathies to the families and others affected by these strikes,” the CJTF-OIR said in a statement.

The U.S.-led coalition has conducted 16,291 airstrikes against ISIS in Iraq and Syria of which 12,633 were conducted by American warplanes. The coalition force periodically probes reports on civilian deaths to determine their credibility and publishes updates on its investigations.

In its statement, the CJTF-OIR said it “investigates all reports of possible civilian casualties using traditional investigative methods, such as interviewing witnesses and examining the site, the Coalition interviews pilots, reviews strike video when available, and analyzes information provided by government agencies, non-governmental organizations, partner forces and traditional and social media. In addition, we consider new information when it becomes available in order to promote a thorough and continuous review process.”

The CJTF-OIR investigated 276 allegations of civilian casualties of which they found 83 of them to be credible.

The U. S. military suffered its first casualty in Syria when a soldier died last week of injuries sustained when an improvised explosive device detonated. The solider, who was part of the CJTF-OIR, was near Ayn Issa in northern Syria when the device exploded.