This photo provided by Hawar News Agency, a Kurdish-run news agency, which has been authenticated based on its contents and other AP reporting, shows fighters from the predominantly-Kurdish Syrian Democratic Forces attending  the funeral procession of 18 of their comrades, who were killed Tuesday by a misdirected airstrike by the U.S.-led coalition, in Tal al-Abyad, northeast Syria, Thursday, April 13, 2017. U.S. Central Command said coalition aircraft were given the wrong coordinates by their partner forces, the Syrian Democratic Forces, for a strike intended to target Islamic State militants south of their Tabqa stronghold. (Hawar News Agency via AP)
This photo provided by Hawar News Agency, a Kurdish-run news agency, which has been authenticated based on its contents and other AP reporting, shows fighters from the predominantly-Kurdish Syrian Democratic Forces attending the funeral procession of 18 of their comrades, who were killed Tuesday by a misdirected airstrike by the U.S.-led coalition, in Tal al-Abyad, northeast Syria, Thursday, April 13, 2017. U.S. Central Command said coalition aircraft were given the wrong coordinates by their partner forces, the Syrian Democratic Forces, for a strike intended to target Islamic State militants south of their Tabqa stronghold. (Hawar News Agency via AP) Credit: Uncredited

Beirut — A misdirected airstrike by the U.S.-led coalition earlier this week killed 18 allied fighters battling the Islamic State group in northern Syria, the U.S. military said Thursday.

U.S. Central Command said coalition aircraft were given the wrong coordinates by their partner forces, the predominantly-Kurdish Syrian Democratic Forces, for the strike on Tuesday that was intended to target IS militants south of their Tabqa stronghold, near the extremists’ de facto capital, Raqqa. The strike hit an SDF position instead.

Several nations have lent their air power to the U.S.-led coalition to defeat the Islamic State group, and it wasn’t clear which air force was behind the errant strike.

The SDF acknowledged the strike, saying a number of its fighters were killed and wounded. On Thursday, the group held funerals for 17 of its fighters in the border town of Tal al-Abyad, the SDF-linked Hawar news agency said, though it did not say whether they were killed in the friendly fire incident.

An activist-run group, Raqqa is Being Slaughtered Silently, said three days of mourning had been declared for the town. The Britain-based Syrian Observatory for Human Rights said 25 SDF fighters were killed in the last two days of battle.

The SDF, meanwhile, announced the launch of a new phase of its campaign to retake Raqqa.

The Kurdish fighters, with U.S.-led air and ground support, have surrounded Tabqa, some 25 miles southwest of the city and are working to clear Islamic State militants out of Jalab Valley, north of Raqqa.

The SDF says it wants to isolate Raqqa before attacking it. Its closest position is less than five miles northeast of the city. But the countryside south of Raqqa is still under IS control.

Meanwhile, President Bashar Assad said a chemical weapons attack on a rebel-held town in northern Idlib province last week that was widely blamed on his forces was a “fabrication.”

“Our impression is that the West, mainly the United States, is hand-in-glove with the terrorists,” Assad told Agence France-Presse in his first comments since a U.S. missile strike on a Syrian air base in retaliation for the chemical attack.

“They fabricated the whole story in order to have a pretext” to attack the air base, Assad said in the interview, a video of which was released by his office.