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.

In announcing the strikes against the Houthis, who have been attacking ships for months in the Red Sea corridor over the Israel-Hamas warin the Gaza Strip, U.S. Defense Secretary Lloyd Austin made a point to offer a warning likely heard in Tehran as well.