US long-range B-2 stealth bombers launched airstrikes early on Thursday targeting underground bunkers used by Yemen’s Houthi rebels, officials said.

It was not immediately clear what damage was done in the strikes.

The Houthis’ al-Masirah satellite news channel reported airstrikes around Yemen’s capital Sanaa, which the group has held since 2014. They also reported strikes around the Houthi stronghold of Saada but offered no immediate information on damage or casualties.

US Defence Secretary Lloyd Austin said the B-2 bombers targeted “five hardened underground weapons storage locations in Houthi-controlled areas of Yemen”.

The strike also appeared to be an indirect warning to Iran, the Houthis’ main benefactor who have twice targeted Israel with ballistic missile attacks over the last year.

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

Neither he nor the US military’s Central Command offered no immediate assessment on the damage done. Central Command said in a statement that initial assessments suggested no civilians had been killed.

It is rare for the B-2 Spirit to be used in the strikes targeting the Houthis, who have been attacking ships for months in the Red Sea corridor over the Israel-Hamas war in the Gaza Strip.

Houthis have targeted more than 80 merchant vessels with missiles and drones since the war in Gaza started last October. They have seized one vessel and sunk two in the campaign that has also killed four sailors.

Other missiles and drones have either been intercepted by a US-led coalition in the Red Sea or failed to reach their targets, which have included Western military vessels.

The rebels maintain that they target ships linked to Israel, the US or the United Kingdom to force an end to Israel’s campaign against Hamas in Gaza. Many of the ships attacked have little or no connection to the conflict, including some bound for Iran.

The Houthis also continue to launch missiles targeting Israel and have shot down a number of US military MQ-9 Reaper drones. The rebels have threatened new attacks after Israel’s ground invasion of Lebanon and its killing of Hezbollah leader Hassan Nasrallah.