Yemen’s Houthi rebels alleged an American airstrike killed at least 68 people and injured scores more in a detention center holding African migrants in northern Yemen on Monday, one of the deadliest attacks in the six-week-long U.

The Houthi militia, which is backed by Iran, said the attack was an American strike. The U.S. Defense Department said it was assessing the claims and took them seriously.

Yemen’s Houthis alleged that a U.S. strike hit a migrant detention center in Saada, killing 68. There was no immediate comment from U.S. Central Command.