(CNN) A Syrian group opposed to the country's autocratic leader, Bashar al-Assad, says airstrikes by the U.S.-led coalition killed 52 civilians.

The Syrian Observatory for Human Rights called the alleged killings a "massacre" committed under the "pretext" of targeting ISIS militants.

The observatory also leveled a fresh charge against the al-Assad regime on Sunday, saying helicopters dropped explosive barrels on a neighborhood -- and that medical sources believe the barrels contained chlorine gas. At last 40 people suffered from suffocation, the group said.

Both attacks allegedly took place Saturday.

The London-based organization said U.S.-led coalition airstrikes hit a village northeast of the city of Aleppo. The dead include seven children and nine women, the observatory said, adding that 13 more civilians were unaccounted for, so the death toll could rise.

CNN could not immediately confirm the report.

U.S. has 'no information to corroborate' the allegation

The U.S. military vowed to investigate.

U.S. Central Command confirmed that coalition forces did conduct airstrikes to the northeast of Aleppo, in the vicinity of Jubb Mahli, on Thursday. It's an earlier date than when the Observatory claims the alleged attack happened.

In the April 30 airstrikes, the coalition destroyed several ISIS fighting positions and hit more than 50 militants, a Central Command spokesman told CNN's Barbara Starr.

Central Command has no indication that any civilians were killed in those strikes, the spokesman said. Kurdish forces had held the town prior to ISIS, and they reported that there had been no civilians in that area for weeks, the spokesman said.

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The U.S. government has cited the observatory's data in the past. For example, in May 2014, the State Department's official blog noted that "The Syrian Observatory for Human Rights currently estimates that some 150,000 Syrians have perished in that country's ongoing conflict."

Group calls for perpetrators to be referred to courts

"We in SOHR condemn in the strongest terms this massacre committed by the U.S led coalition under the pretext of targeting the IS in the village," the observatory's statement said Saturday, "and we call the coalition countries to refer who committed this massacre to the courts, as we renew our calls to neutralize all civilians areas from military operations by all parties."

The U.S.-led military coalition said Friday that it had launched 18 air strikes against ISIS, which calls itself the Islamic State, in Syria and Iraq since early Thursday. The coalition, formed in October 2014 to fight the ISIS terrorist group, includes Bahrain, Jordan, Saudi Arabia, and the United Arab Emirates.

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Latest in a string of allegations that Syria uses chlorine gas

The group's complaint about the Syrian regime's alleged chlorine gas attack was the latest in a string of similar accusations.

Two explosive barrels fell on the northern neighborhood of Saraqib, the observatory said. Medical sources reported about 40 cases of suffocation in civilians, including children, who inhaled gases from the explosive barrels, the observatory added.

Medical sources believe the barrels contained the gas, the observatory said.

CNN could not verify the claims independently.

Syria's state-run news agency SANA has not reported on an operation in that area on Saturday. It carried several reports of Syrian military forces cracking down on "terrorists" in the country.

A fearsome toll

Syria has been wracked by civil war for four years, with a fearsome toll of death, destruction and refugees.

SOHR has in the past been accused of selective reporting of the atrocities, publicizing only those committed by the al-Assad regime and ignoring those committed by his opponents. It has also been criticized as essentially a one-man show, run in Britain by a man who left Syria more than 10 years ago, and for failing to make its methodology public.

The situation in Syria is complicated. The U.S. has called for al-Assad to leave power. Yet is it conducting airstrikes against ISIS, which is also opposed to al-Assad.