The US officially entered the conflict in Yemen with a salvo of cruise-missile strikes on three coastal radar sites in areas of Yemen controlled by Houthi rebels, retaliating after failed missile attacks this week on a US Navy destroyer, US officials said late Wednesday.

There was no immediate word of any casualties in Yemen.

US officials told Reuters that the Arleigh Burke-class USS Nitze launched the Tomahawk cruise missiles at about 4 a.m. local time on Thursday.

"These radars were active during previous attacks and attempted attacks on ships in the Red Sea," US officials told USNI News' Sam LaGrone, adding that the radar sites were in remote areas with a low risk of civilian casualties.

"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," Michael Knights, an expert on Yemen's conflict at The Washington Institute for Near East Policy, told Reuters.

The strikes authorized by President Barack Obama represent Washington's first direct military action against Houthi-controlled targets in Yemen's conflict. The Pentagon said initial US assessments indicated the radar sites were destroyed.

"These limited self-defense strikes were conducted to protect our personnel, our ships, and our freedom of navigation in this important maritime passageway," Pentagon spokesman Peter Cook said. "The United States will respond to any further threat to our ships and commercial traffic, as appropriate."

Two times in the past four days the USS Mason, a guided-missile destroyer, fired interceptor missiles in self-defense after detecting incoming missiles launched from the territory held by Iranian-backed Houthi militants. The incidents occurred in the Bab al-Mandab Strait between Yemen and Eritrea with no damage or injuries to the US Navy.

The Houthis, a militant uprising against the internationally recognized government of Yemeni President Abd Rabbu Mansour al-Hadi, denied previous attempts on the USS Mason, but had taken credit for a similar missile strike that savaged a former US Navy ship operated by the United Arab Emirates.

The direct strike against the Houthis makes the US a participant in the conflict in Yemen, where Saudi Arabia has led a brutal air campaign linked to high civilian deaths and potentially war crimes.

Reuters previously reported that the US had worried that direct involvement in the conflict on Saudi Arabia's behalf could make it liable to be tried for possible war crimes committed during the 18-month conflict.

Forensic experts seen Sunday at the community hall where Saudi-led warplanes struck a funeral in Sanaa, the capital of Yemen. Khaled Abdullah/Reuters

Simultaneously, the US has been trying to manage a fragile relationship with Iran after the adoption of the Joint Comprehensive Plan of Action to curb Iran's nuclear program.

Iran backs the Houthi militants, has provided them with arms, and previously has openly provoked the US Navy in international waters.