U.S. B-2 bombers and other aircraft have hit five underground Houthi weapons storage sites in parts of Yemen the Iran-backed rebel group controls, Defense Secretary Lloyd Austin said Wednesday.

The facilities, which officials said were hardened, housed “several weapons components of the type that the Houthis have used to attack civilian and military vessels throughout the region,” Austin said.

Houthi rebels have attacked civilian ships in the Red Sea and the Gulf of Aden. The group was suspected of an attack in August.

The strikes marked the first time the U.S. used B-2 bombers, also known as stealth bombers, in attacks against the Houthis in Yemen, a U.S. official told NBC News.

“This was a unique demonstration of the United States’ ability to target facilities that our adversaries are trying to keep out of reach, no matter how deeply underground, hardened or fortified,” Austin said in a statement.

The Houthis, an Iranian-backed militia that has taken over part of Yemen, have begun launching rockets and drones and carrying out other attacks on shipping vessels in response to the war in Gaza. The Houthis have expressed their support for the terrorist group Hamas.

A $1 trillion cargo flow flows through the Red Sea every year. Some shippers responded to the attacks late last year by suspending services in the Red Sea.

The US first launched airstrikes against Houthi weapons in January in response to Houthi attacks on commercial shipping.

Austin said he authorized Wednesday’s attacks at the direction of President Joe Biden.

Austin said the strikes are intended “to further reduce the Houthis’ ability to continue their destabilizing behavior and to protect and defend U.S. troops and personnel in one of the world’s most critical waterways.”

The Houthis captured Yemen’s capital Sana’a in 2014. The ongoing conflict has caused extreme hardship. The United Nations calls the humanitarian crisis in Yemen the world’s worst and says more than 18 million people are dependent on humanitarian aid.