A Russian and Syrian airstrike intended for Islamic State in Iraq and Syria (ISIS) militants mistakenly hit U.S.-backed fighters in Syria, a top U.S. general said Wednesday.

Russian and Syrian regime aircraft on Tuesday bombed villages southeast of Syrian city al-Bab that they thought were held by ISIS, Army Lt. Gen. Stephen Townsend said during a briefing.

"Yesterday, we had some Russian aircraft and regime aircraft bomb some villages that I believe they thought were held by ISIS,” Townsend said. “Yet, they were actually — on the ground were some of our Syrian Arab Coalition forces."

Townsend confirmed that there were casualties among Syrian fighters as a result, though he did not specify how many.

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U.S. forces were in the area — about three miles away — when the airstrikes took place, though no Americans were hit.

There are a number of different fighters in northern Syria, including Syrian government forces, Turkish-backed Syrian fighters and Kurdish fighters.

Townsend said the various forces make the current situation very complicated, and urged all parties to focus on fighting ISIS.

“Everybody should keep their sights focused on ISIS, and that's what we ought to keep our efforts focused on and not fighting deliberately or accidentally with one another,” he said.

Currently, about 500 U.S. troops are in Syria but are not involved in direct combat and are away from the front lines of the fighting.

The incident on Tuesday marked the second time in a month that Russian aircraft mistakenly attacked friendly forces. In early February, a Russian bombing killed three Turkish soldiers in northern Syria.