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An airstrike by a Saudi-led coalition fighting Yemen's Iran-aligned Houthi rebels in Yemen that killed dozens of people in Yemen last month is an "apparent war crime," an international rights group said Sunday.

The coalition backing Yemeni internationally recognized government expressed on Saturday its regret and pledged to hold to account those responsible for the airstrike, which hit a bus carrying children in a busy market. At least 51 people, including 40 children, were killed, and 79 others, including 56 children, were wounded in the airstrike in the northern province of Saada.

Human Rights Watch said that the attack adds to the coalition's "already gruesome track record of killing civilians at weddings, funerals, hospitals, and schools in Yemen."

The New York-based group said it spoke by phone to 14 witnesses, including 9 children, who said that shortly before 8:30 a.m. on August 9, an aerial bomb hit the market in Dhahyan, a town 20 kilometers north of Saada in Houthi-controlled northwestern Yemen, 60 kilometers from the Saudi border.

It said the bomb landed within a few meters of a bus filled with boys on an excursion organized by a local mosque to visit the graves of men who had been killed in fighting. The bus was parked outside a grocery store where the driver had gone to buy water for the children, HRW said.

Bill Van Esveld, senior children's rights researcher at the group, has urged the U.S. and other countries to "immediately stop weapons sales to Saudi Arabia and support strengthening the independent UN inquiry into violations in Yemen or risk being complicit in future atrocities."

The coalition said it has accepted the conclusions of its investigative body, known as the Joint Incidents Assessments Team, which found that the August 9 airstrike involved "mistakes," including failing to take measures to minimize collateral damage.

The coalition said in Saturday's statement "it will take all the legal measures to hold accountable those who were proven to have committed mistakes" once it officially receives the findings. It also pledged to coordinate with Yemen's government to compensate civilians.

The U.S. State Department welcomed on Sunday the Saudi-led coalition's ?announcement as "an important first step toward full transparency and accountability." It called on all sides of the conflict to "abide by the Law of Armed Conflict, to mitigate harm to civilians and civilian infrastructure, and thoroughly investigate and ensure accountability for any violations."

Yemen's civil war pitting the Saudi-led coalition against Iran-aligned rebels, known as Houthis, has been raging for more than three years. The coalition backs Yemen's internationally recognized government of Abed Rabbo Mansour Hadi and aims to restore it to power.

The coalition has faced criticism for its campaign of airstrikes that has killed civilians and destroyed hospitals and markets. The Houthis, meanwhile, have been accused of using land mines, killing and wounding civilians. They have also targeted religious minorities and imprisoned opponents.

The rebels also often attempt cross-border attacks on Saudi Arabia and have targeted the kingdom's vessels in the Red Sea.

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Associated Press writer Susannah George in Washington contributed to this report.