ISTANBUL, TURKEY - U.S. military officials are assessing whether to open an investigation into a coalition airstrike earlier this week in northern Syria that a monitoring group and locals claim killed at least eight civilians including children, among them five sisters.

The airstrike on Tuesday on the northern Syrian village of Atme appeared to have been targeting a weapons depot belonging to the Jaish al-Sunna militia, which is part of a rebel alliance that includes al-Qaida affiliate Jabhat al-Nusra, say local activists.

US May Investigate Claims of Civilian Deaths in Coalition Airstrikes

The U.S. has mounted several attacks on al-Nusra in the past, specifically targeting al-Qaida veterans.

US May Investigate Claims of Civilian Deaths in Coalition Airstrikes

Islamist rebel commanders who coordinate with al-Nusra suggested the coalition airstrike may have been a warning to them to discontinue an assault on the nearby Kurdish border town of Afrin in neighboring Aleppo province. Last week, al Nusra fighters and allies stormed a district of the town and engaged in fierce firefights with Kurdish defenders, who fear al-Nusra may soon launch suicide bombings on the town.

US May Investigate Claims of Civilian Deaths in Coalition Airstrikes

At first U.S. officials denied having launched an airstrike on the village just north of Idlib where a refugee camp housing more than 20,000 refugees is also located. But later they confirmed the raid, saying there had been confusion over the spelling of the village’s name, which in English can also be rendered as “Atmeh,” “Atma,” or “Atima.”

Later, in a statement, U.S. Central Command said: “We are aware of the reports surrounding Atmeh and are looking into them. We take all reports of non-combatant casualties seriously and look into each one received or reported. We apply very rigorous standards in our targeting process to avoid or minimize civilian casualties in the first place—to include analysis of available intelligence to selection of the appropriate weapons to meet mission requirements with the goal to minimize the risk of collateral damage and, in particular, potential harm to non-combatants.”

At least 18 people were killed in the airstrike; ten were militiamen, claim activists. “Eight were civilians, including five children and two women,” said Rami Abdul Rahman of the Syrian Observatory for Human Rights, a UK-based monitoring group that relies on political activists inside Syria for its information. Residential buildings near the target were also struck. “The strikes hit the Jaish al-Sunna weapons depot, which was also used to manufacture shells and rockets. There was a huge explosion after the strikes,” he said.

The five children reportedly killed in the attack were sisters aged between 4 and 10 years and activists posted their photographs online. Their mother survived the attack but was seriously injured.

Local relief workers reported on social media Wednesday that four children may still be buried in rubble. Hadi al-Abdullah, a Syrian journalist working on the ground, interviewed locals and posted his report on YouTube. Locals told him a total of six missiles impacted the weapons depot and nearby buildings. In one interview a man who says he is the grandfather of the sisters reported dead complains, “Are these girls terrorists? Are they carrying machine guns? May God take revenge!”

Syrian rebel commanders claimed Tuesday’s sortie was flown by U.S. warplanes that took off from the NATO base at the ?ncirlik Air Base in southern Turkey. Turkish authorities have only recently allowed U.S. fighters, tasked with bombing missions inside Syria, to fly from the base and the first detachment of F-16s arrived last Sunday.

The Turkish foreign ministry denied the warplanes for the airstrike on Atme came from ?ncirlik.

The Jaysh al-Sunna militia is a small independent unit that was first raised in Homs. It has links with the Western and Gulf-backed Free Syrian Army. In March it also joined the Army of Conquest (Jaysh al-Fatah), an insurgent alliance that includes al Nusra and hardline Islamist militias such as Ahrar al-Sham. The alliance has made significant advances in Idlib province, driving out Syrian government forces from strategic towns. In June the alliance reinforced mainly FSA militias north of Aleppo to stem an offensive by Islamic extremists.

The airstrike has prompted a fierce reaction from Islamist militias and political activists and analysts warn it will likely add to the challenges for Washington and Ankara in shaping a rebel defense force to protect a coalition planned buffer zone in northern Syria.