This photo depicts an explosion after an apparent airstrike carried out against Islamic State militants near Kobane, Syria, in October. EPA/SEDAT SUNA

The U.S. military is investigating credible reports of civilian casualties in its campaign against Islamic State militants, the Pentagon press secretary said Tuesday, a shift after months in which defense officials said they were aware of none.

Rear Adm. John Kirby disclosed the probes on Tuesday, telling reporters at the Pentagon that U.S. Central Command is leading the review. CENTCOM officials said they have investigated the credibility of 18 separate allegations of coalition airstrikes resulting in civilian casualties between Aug. 8 and Dec. 30, and determined 13 were not credible.

Five more remain under review, however, including two from late December determined to be credible. They began as “the direct result of our own internal review process” said Army Maj. Curtis J. Kellogg, a CENTCOM spokesman, in a statement. The review of the other three allegations are still in initial phases, he added.

While the military had previously acknowledged reviewing reports of civilian casualties in airstrikes in Iraq and Syria, the elevation of two of those incidents — one in Iraq and one in Syria — into formal investigations had not previously been disclosed.

“The key to any allegation is whether sufficient verifiable information is available to make a determination,” Kellogg said. “A source is generally deemed to be credible if the source provides verifiable information, such as corroborating statements, photographs or documentation that can help us determine whether an allegation is founded.”

The U.S.-led coalition fighting the Islamic State has carried out about 1,400 airstrikes since they began Aug. 8, including some in urban areas. The eighteen reports of civilian casualties came from sources that includes internal reviews, unit self-reporting, media reports, non-governmental organizations and other U.S. government agencies like the State Department, Kellogg said. Nine were reported in Syria and nine were reported in Iraq, Kellogg said.

Army Lt. Gen. James Terry, the top commander of the task force overseeing operations in Iraq and Syria, told reporters Dec. 18 at the Pentagon that he was unaware of any civilian casualties. The coalition is deliberate in efforts to avoid them, he said, citing a desire to avoid a “bad situation” in which Iraqi security forces or tribesmen working with the Iraqi government are hit.

Terry said then that the United States is able to carry out airstrikes without American troops on the ground because of coordination with the Iraqi military and U.S. surveillance capabilities.

“We bring the right people into that to actually help us identify units, and then what we call deconflict of fires and the clearance of fires, so there are Iraqis in the process when we do all this,” Terry said.

Kellogg said Tuesday that the recent reports deemed to be credible occurred after Terry’s remarks.

Any proven civilian casualties could raise questions anew about how the United States is carrying out airstrikes in Iraq and Syria.

In Iraq, the United States has relied on communication with Iraqi troops, with U.S. troops directing coalition pilots from joint operations centers in Baghdad and the northern city of Irbil. In carrying out close-air support, however, the military traditionally relies on joint terminal attack controllers, who coordinate the strikes, work alongside infantrymen and other combat troops on the ground to guide pilots’ bombs on target. Without JTACs on the ground, doing so is considerably more complicated.

In Syria, the U.S. military has even less intelligence to work with, other than in the northern part of the country where it coordinates with Kurdish peshmerga militias that have defended the town of Kobane and surrounding areas. Most coalition airstrikes in Syria have occurred there, but not all. The independent Syrian Observatory for Human Rights reported in October that while most of those killed in the airstrikes have been militants, numerous civilians have been killed as well.

This post has been updated with additional reporting.