An image taken from video footage and released by Russia’s Defense Ministry on Aug. 18, 2016, shows a Russian fighter-bomber dropping bombs in Syria. (Ministry of Defense of the Russian Federation via Reuters)

Russian and Syrian regime forces bombed U.S.-backed fighters Tuesday in villages near the Syrian city of al-Bab, a U.S. general said Wednesday, characterizing the incident as a likely mistake.

Army Lt. Gen. Stephen Townsend, who leads the fight against the Islamic State in both Iraq and Syria, told reporters Wednesday that he believes the pilots thought they were bombing militants, not the Syrian Democratic Forces that the United States trains and advises. U.S. forces, who were a couple of miles away to assist the Syrian fighters, called in the information and the Russians halted the airstrikes, Townsend said.

Russia denied any involvement in the bombings afterward, saying neither Russian nor Syrian aircraft were involved, according to the Kremlin-funded news agency RT. “Not a single” airstrike on the areas Townsend cited were carried out by the two militaries, Russian defense officials said in a statement. Russia often denies involvement in military operations despite evidence to the contrary.

Townsend said the bombings occurred south and east of al-Bab. Islamic State fighters had been in the area but had left to get away from advancing forces, including rebels backed by Turkey.

“The ISIS fighters withdrew, and the Syrian Arab Coalition fighters advanced into those villages,” Townsend said of the fighters who were targeted, using a common acronym for the Islamic State.

Townsend said the incident illustrates how complicated the battlefield has become near al-Bab, which is less than 20 miles from the Turkish border in northwest Syria and was seized from the Islamic State by Turkish-backed forces last month. Syrian fighters backed by the United States, the Turks as well as Syrian forces backed by Russia all are still active in the area.

The United States and Russia established a hotline in 2015 in which a U.S. colonel in Qatar and a Russian colonel in Syria work to “deconflict” operations and prevent aerial collisions. But senior U.S. officials have not cooperated directly with the Russians, in part because of legislation first passed in 2014 that forbids that kind of engagement because of Russia’s annexation of Ukraine’s Crimean Peninsula. Senior U.S. military officials in the Middle East have advocated for increased deconfliction talks, citing the close proximity in which the militaries are now operating in Syria.

Townsend said Wednesday that the deconfliction hotline worked to stop the bombing, and that in al-Bab different kinds of forces are “literally within hand-grenade range of each other.”

“This is something that goes on daily in the air,” Townsend said of deconfliction. “Not every day on the ground, but daily in the air there’s a deconfliction arrangement with the Russians. And we used that mechanism and it worked.”

The incident comes as the White House mulls how to accelerate the war against the Islamic State. Townsend said he has made his recommendations to the Pentagon but declined to discuss them. They are expected to include new deployments for some U.S. troops.

This story was updated to include Russia’s denial of the bombings.

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