A Saudi Arabia-led military coalition said “mistakes were made” after it killed more than 40 children in an airstrike in northern Yemen last month that struck a school bus returning from a summer field trip.

Most of the children were under the age of 10. CNN determined that the bomb that killed the children was American made, and had been sold to Saudi Arabia as part of a State Department-sanctioned arms deal.

An internal coalition investigation “concluded that there were mistakes made in abiding by the rules of engagement,” said a statement issued by the coalition Saturday, The Washington Post reported. It expressed “regret for these mistakes” and offered “condolences and solidarity with the families of the victims.” The statement pledged to hold those responsible accountable, and said families of the victims would be compensated.

The coalition initially characterized the airstrike in Saada province as a “legitimate military action” against Iran-allied Houthi rebels who had fired a ballistic missile into southwestern Saudi Arabia. It claimed the rebels were inside the school bus and using children as human shields.

The military coalition is battling the rebels to restore the internationally recognized government of President Abd-Rabbu Mansour Hadi, who was ousted from the capital by rebels in 2015.

The United Nations blasted the coalition in a report Tuesday for the high civilian death toll in airstrikes in Yemen. It warned that some of the strikes “may amount to war crimes,” Reuters reported.

In the past three years, “such airstrikes have hit residential areas, markets, funerals, weddings, detention facilities, civilian boats and even medical facilities,” the report said. Its investigation of 11 strikes raised “serious concerns” about the coalition’s “targeting process.”

After the U.N. report was issued, U.S. Defense Secretary James Mattis warned that American support is not unconditional, and called for more precautions to protect civilians, CNN reported.

The coalition has purchased billions of dollars in weapons from the Pentagon, which also shares intelligence with the group.

U.N.-sponsored negotiations are being held in Geneva next week in an effort to resolve the two-year-old conflict.