The U.S. military confirmed Thursday a "misdirected" airstrike from the U.S.-led coalition this week killed 18 allied fighters battling the Islamic State terror network in northern Syria.

Syrian Democratic Forces, partnering with the U.S., gave coalition aircraft the wrong coordinates for a strike intended to hit ISIS south of its Tabqa stronghold, according to U.S. Central Command.

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The strike hit an SDF position instead, killing 18 fighters on Tuesday, the U.S. officials said. "The Coalition's deepest condolences go out to the members of the SDF and their families."

It was the third time in a month U.S.-led airstrikes may have killed allies or civilians, The New York Times noted.

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The SDF, with U.S.-led air and ground support, has surrounded Tabqa, 25 miles southwest of Raqqa. They say they are working to clear ISIS fighters out of Jalab Valley, north of Raqqa.

It was not clear which air force was behind the strike. Several nations have lent their air power to the coalition to defeat ISIS.

"The Coalition is assessing the cause of the incident and will implement appropriate safeguards to prevent similar incidents in the future," U.S. Central Command added.

The SDF-linked Hawar News Agency reported the group was holding funerals for 17 of its fighters in the border town of Tal al-Abyad, though it did not link them to the strike. An activist-run group that calls itself "Raqqa is Being Slaughtered Silently" said the town has gone into three days of mourning. 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 fourth phase of its campaign to capture Raqqa, a Euphrates River city that is home to 300,000 people.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.