A Saudi-led military coalition said Saturday it made mistakes in an airstrike last month that killed dozens of Yemeni schoolchildren, a rare admission of guilt for the U.S.-backed alliance.

The coalition, which is fighting a war against Yemen’s Iran-supported Houthi rebels, apologized for the strike and promised to hold accountable those responsible, pending the official findings from a Saudi-based investigatory body, according to a statement.

The coalition would also revise its rules of engagement “in a manner that guarantees non-recurrence of such incidents,” it said.

Saudi Arabia and its mainly Arab allies have carried out airstrikes and a ground campaign in Yemen for more than three years in a bid to oust the Houthis, who hold power in the capital San’a. They want to restore the internationally backed government of President Abed Rabbo Mansour Hadi.

But as the human toll mounts, the U.S.-backed coalition’s airstrikes have come under intensifying scrutiny from human-rights groups, the United Nations and Washington. The conflict overall has killed more than 6,660 civilians, according to the U.N., a large portion of them allegedly by coalition strikes.