AL-BAB, SYRIA — A U.S. general said on Wednesday that Russian and Syrian regime forces have accidentally bombed U.S.-backed forces in northern Syria.

The incident took place southeast of the city of al-Bab. U.S. Army Lt. Gen. Stephen Townsend told reporters that Islamic State fighters withdrew from the villages before the U.S.-backed Syrian Arab Coalition fighters advanced.

“We had some Russian aircraft and regime aircraft bomb some villages that I believe they thought were held by ISIS, yet ... actually on the ground were some of our Syrian Arab Coalition forces,” AFP quoted Townsend as saying. Townsend did not comment on the actual casualties caused by the airstrikes.

According to the Kremlin-funded news agency RT, Russia denied any involvement in the bombings.

“Not a single air strike on the regions specified by the US side was conducted by Russian or Syrian air forces,” RT quoted from a statement issued by Russia’s Defense Ministry.