Jim Michaels

USA TODAY

WASHINGTON — Russian aircraft mistakenly struck U.S.-backed forces in northern Syria, a U.S. general said Wednesday.

The airstrikes resulted in some casualties among the Syrian Arab force, said Lt. Gen. Stephen Townsend, commander of coalition forces supporting Iraqi and Syrian forces battling the Islamic State. He did not say how many people were injured.

The Russians likely believed they were hitting Islamic State targets and ceased the strikes on Tuesday once they were notified by U.S. forces, Townsend told reporters during a video press briefing.

U.S. advisers who are supporting the Syrian forces were several miles away when the Russians launched the airstrike.

The mistake in northern Syria highlights the complexity of the Syrian battlefield, where numerous forces with differing agendas are engaged in fighting. Russia, Turkey, the U.S. and other countries are backing surrogate forces inside Syria, which has been in a civil war since 2011.

The U.S. is backing a force of Syrian Arabs and Kurds that is closing in on Raqqa, the de-facto capital in Syria of the Islamic State, which is also called ISIS.

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Townsend urged all factions to work together to focus on defeating the Islamic State. "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," Townsend said.

To avoid mishaps in the air the U.S. and Russian militaries are notifying each side about where they are operating, but they are not coordinating by sharing intelligence and targeting information.

The need for these so-called "deconfliction" talks are becoming more critical as ISIS increasingly loses ground and American and Russian aircraft are operating in a small area.

“As we continue to squeeze ISIS and continue to take more territory away from them, it becomes more complex," Gen. Herbert Carlisle, head of Air Combat Command, said recently.