The U.S. military unleashed B-2 stealth bombers to target underground bunkers used by Yemen’s Houthi rebels early Thursday, a major escalation in the American response to the rebels’ attacks on Mideast shipping lanes that appeared to be a warning to Iran as well.

While it wasn’t immediately clear how much damage the strikes caused, the attack appeared to be the first use of the B-2 in combat in years and the first time the flying wing targeted sites in Yemen.