The U.S. Department of Defense on Tuesday announced that it had initiated investigations into allegations that a large number of civilians were killed in airstrikes by the U.S.-led coalition during its campaign against the Islamic State group in Iraq and Syria. This is the first time that the Pentagon has acknowledged that airstrikes in these countries may have led to civilian deaths.

“What I know is that CENTCOM, Central Command, is investigating several, what they believe to be credible allegations of possible civilian casualties,” Rear Adm. John Kirby, the Pentagon press secretary, said, during a press briefing on Tuesday. “They are actively investigating what they believe to be at least a few incidents of civilian causalities that they think warrant further investigation, that they have found credible to investigate.”

CENTCOM officials told The New York Times that the military is currently investigating two instances of civilian deaths -- one in Iraq and one in Syria -- after examining the credibility of 18 allegations. The official also told the Times, without providing further details, that of the 18 allegations, 13 were found to lack credibility.

The announcement by the U.S. government comes just days after organizations monitoring the war in the region claimed that over 33,000 civilians had been “killed by all sides” in Iraq and Syria in 2014. The United Nations had also lamented the “very sad state of affairs” in the region, and condemned the high number of civilian casualties in the war against ISIS.

“We are very mindful of trying to mitigate the risk to civilians every time we operate, everywhere we operate. And so when we do believe that we've had occasion to cause collateral damage or hurt, kill civilians, we take it seriously and we look into it,” Kirby said.

The U.S.-led coalition has carried out nearly 1,400 airstrikes on ISIS strongholds in Iraq and Syria since the aerial offensive began last August, according to media reports. Until now, the Pentagon had maintained that it had not received any credible reports of civilian casualties caused by U.S. bombs.