Story highlights The troops were lightly wounded on June 9

The administration hasn't publicly revealed the incident

(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.

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