American forces hit an undersea drone during five "self-defense strikes" against Houthi rebels in areas of Yemen controlled by the Iran-backed rebels, U.S. Central Command said Sunday.

Why it matters: This is the first time U.S. forces have observed Houthis using an unmanned underwater vessel (UUV) since the group began attacking merchant and commercial ships passing through the Red Sea on Oct. 23, per a CENTCOM statement on Saturday's strikes.

Details: U.S. forces successfully struck the UUV, three mobile anti-ship cruise missiles and one unmanned surface vessel between 3-8pm Saturday Yemeni time, according to the statement.

• CENTCOM had determined these five targets in Houthi-controlled areas of Yemen "presented an imminent threat to U.S. Navy ships and merchant vessels in the region," per the statement.
• "These actions will protect freedom of navigation and make international waters safer and more secure for U.S. Navy and merchant vessels."

The big picture: The Houthis said they began the Red Sea boat assaults in protest against the Israel-Hamas war and claimed they were targeting ships linked to Israel. Many of the ships they've attacked have no link to Israel.

• The Biden administration responded to the missile and drone attacks on ships by establishing a multinational maritime security force in December.
• The U.S. and U.K. have conducted several strikes on Houthi targets this year in response to further attacks on vessels in the Red Sea.

What they're saying: Vice Adm. Brad Cooper, deputy commander of U.S. Central Command, told CBS' "60 Minutes" in an interview broadcast Sunday that "15% of global trade flows exactly through the Red Sea," so "keeping these vital waterways open is critical."

• It's "a core commitment the United States has from a strategic perspective, maintaining the free flow of commerce," he added.

Between the lines: Mick Mulroy, a former Pentagon official and CIA officer, told the New York Times the Houthis' use of an underwater drone was notable and it seemed the group was changing its approach.

• "Unmanned surface and subsurface vessels are likely more difficult to detect and destroy than aerial drones and anti-ship missiles," Mulroy added.
• "If all of these weapons systems were used against one target, it could overwhelm the ship's defenses."

Go deeper: U.S. puts Yemen's Houthi rebels back on global terrorist list

Editor's note: This article has been updated with new details throughout.