(CNN) Four U.S. military advisers were lightly wounded earlier this month in northern Syria by attackers who were believed to be part of ISIS, CNN has learned.
The Pentagon has not yet spoken publicly about the June 9 incident, but several defense officials told CNN it happened when an anti-tank round was fired close to the advisers' position and exploded a vehicle near them.
The troops suffered light shrapnel wounds and are believed to have returned to duty. The officials also did not immediately know if the troops involved returned fire at the attackers.
While the attackers are believed to be ISIS, the U.S. military is not certain of their identity.
The administration's official policy is that U.S. troops are in Syria and Iraq for training, advising and assisting local forces, but that their role is not to engage in combat. They have returned fire when attacked, and have in the past moved in to assist local forces when they are under fire.
On at least five occasions, American troops have either been wounded or killed in Iraq and Syria when faced with enemy fire while training and assisting U.S.-backed forces.
The Pentagon generally does not publicly disclose when troops are wounded, officials said. Because these incidents mainly involve U.S. special operations forces, the military does not want to disclose the specific locations in which they are operating, in hopes of keeping their locations secret from ISIS.
However, when troops are killed in action, there are policies that require the Pentagon to disclose the name of the killed service member and the location where it happened.
Defense officials also said they do not disclose specific information about the health status of individual troops who have been wounded due to privacy concerns. And the officials said they may not know of every incident in which troops fire their weapons at attackers because some incidents may not rise to the level of importance of being reported to the Pentagon.