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U.S. bombs were used in Saudi airstrikes on a market in Yemen that killed around 100 people — including two dozen children — a New York-based human rights group said Thursday.

Human Rights Watch said its investigators found remnants of American-supplied parts at the site of the Mar. 15 bombing.

Secretary of State John Kerry said he didn't have any "solid information" or documentation on what weapons were used in the attack. "There are questions being asked … Whatever weapons are being used, our preference is that all shooting stops," he told Reuters and other reporters.

A Saudi-led coalition backed by the U.S. has been fighting Shiite Houthi rebels in Yemen since March last year. Although the warring parties have agreed to a ceasefire starting Sunday, HRW said the U.S.-backed group has "carried out numerous indiscriminate and disproportionate airstrikes."

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Shiite rebels, known as Houthis, protest against Saudi-led airstrikes, in Sanaa, Yemen, on March 18. AP

The U.N. reported that two bombs hit the market in the northwest town of Mastaba on Mar. 15, killing at least 96 people, including 24 children. United Nations staff who visited the site counted later another 10 bodies burned beyond recognition and described the scene as "carnage."

A witness told HRW he saw about 10 Houthi fighters among the bodies, and that some of the militants regularly ate and slept around 60 yards from where one bomb detonated.

"One of the deadliest strikes against civilians in Yemen’s year-long war involved US-supplied weapons"

"The only Houthi military presence identified by Human Rights Watch during its visit was a checkpoint manned by two or three fighters about 250 meters north of the market," HRW added.

A day after the attack, Saudi military spokesman Gen. Ahmad al-Assiri said the strike targeted a "a militia gathering," according to HRW.

NBC News was unable to immediately reach the Saudi ministry of defense for comment on Thursday's report.

HRW said its team conducted on-site investigations on Mar. 28 and found remnants of a satellite-guided bomb whose components were supplied by the U.S.

"One of the deadliest strikes against civilians in Yemen’s year-long war involved U.S.-supplied weapons, illustrating tragically why countries should stop selling arms to Saudi Arabia," Priyanka Motaparthy, emergencies researcher at Human Rights Watch, said in a statement Thursday.

"The U.S. and other coalition allies should send a clear message to Saudi Arabia that they want no part in unlawful killings of civilians," she added. HRW called on the U.S., U.K., France and others to "suspend all weapons sales to Saudi Arabia until it curtails its unlawful airstrikes in Yemen."