The US military launched cruise missile strikes on Thursday to knock out three coastal radar sites in areas of Yemen controlled by Iran-aligned Houthi forces, retaliating after failed missile attacks this week on a U.S. Navy destroyer, US officials said.

The strikes, authorized by President Barack Obama, represent Washington’s first direct military action against Houthi-controlled targets in Yemen’s conflict.

“These limited self-defense strikes were conducted to protect our personnel, our ships and our freedom of navigation,” Pentagon spokesman Peter Cook said.

U.S. officials, speaking on condition of anonymity, said U.S. Navy destroyer USS Nitze launched the Tomahawk cruise missiles around 4 a.m. (0100 GMT). “These radars were active during previous attacks and attempted attacks on ships in the Red Sea,” including the USS Mason, one of the officials said, adding the sites were in remote areas where the risk of civilian casualties was low.

“Targeting US warships is a sign that the Houthis have decided to join the axis of resistance that currently includes Lebanese Hezbollah, Hamas and Iran,” Knight said.

The Houthis, who are allied to Hadi’s predecessor Ali Abdullah Saleh, have the support of many army units and control most of the north, including the capital Sanaa.

“The United States will respond to any further threat to our ships and commercial traffic, as appropriate,” Cook said.

Reuters has reported that the coastal defense cruise missiles used against the USS Mason had considerable range, fuelling concern about the kind of weaponry the Houthis appear willing to employ and some of which, U.S. officials believe, is supplied by Iran.