WASHINGTON 

A Houthi unmanned surface vessel detonated in the Red Sea on Thursday, a US navy commander said, adding that there were no casualties.

"Since November 18, there have now been 25 attacks against merchant vessels transiting the Southern Red Sea and Gulf of Aden.

"The latest was early this morning when a Houthi one way attack, unmanned surface vessel or USV detonated in international shipping lanes," Vice Admiral Brad Cooper told reporters.

Cooper said there were no casualties and no ships were hit, but the new tactic is "of concern."

Yemen’s Houthi have targeted vessels in the Southern Red Sea, warning it will attack all Israel-bound ships. It says the attacks are to support Palestinians as they face Israel's "aggression and siege" in Gaza.

The Red Sea is one of the world's most frequently used sea routes for oil and fuel shipments.

US Defense Secretary Lloyd Austin recently announced the creation of a multinational mission -- Operation Prosperity Guardian -- to counter the Houthi attacks.

Cooper said this operation is "entirely defensive in nature," and the contributions of participant countries are "meaningful" and the partners are doing "great work" at sea.

About 1,500 merchant ships safely transited the waters of the Red Sea since the operation began and collaboration with the maritime shipping industry has increased dramatically, he said.