WASHINGTON (REUTERS) - The US military launched cruise missile strikes on Thursday (Oct 13) 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 US Navy destroyer, US officials said.

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

Still, the Pentagon appeared to stress the defensive nature of the strikes, which were aimed at radar that enabled the launch of at least three missiles against the US Navy destroyer USS Mason since Sunday.

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

US officials, speaking on condition of anonymity, said US Navy destroyer USS Nitze launched the Tomahawk cruise missiles around 4 am local time (9am Singapore time).

"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 that the sites were in remote areas where the risk of civilian casualties was low.

The official identified the areas in Yemen where the radar were targeted as near Ras Isa, north of Mukha and near Khoka.

The missile attacks on the USS Mason - the latest of which took place earlier on Wednesday - appeared to be the Houthis'response to a suspected Saudi-led strike on mourners gathered in Yemen's Houthi-held capital Sanaa.

Dr Michael Knights, an expert on Yemen's conflict at the Washington Institute for Near East Policy, suggested the Houthis, fighters from a Shi'ite sect, could be becoming more militarily aligned with groups such as Lebanon's Shi'ite militant group Hezbollah.

"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," he said.

The missile incidents, along with an Oct 1 strike on a vessel from the United Arab Emirates, add to questions about safety of passage for military ships around the Bab al-Mandab Strait, one of the world's busiest shipping routes.

The Pentagon warned against any future attacks. "The United States will respond to any further threat to our ships and commercial traffic, as appropriate," Mr Cook said.