This handout photo shows the Arleigh Burke-class guided-missile destroyer USS Carney (DDG 64) defeating a combination of Houthi missiles and unmanned aerial vehicles in the Red Sea. AFP

American forces targeted 14 missiles that were ready to launch in Yemen, the US military said Wednesday, after Washington re-designated the Iran-backed Houthi rebels as a "terrorist" entity for their attacks on merchant vessels.

The Houthis — who have already faced multiple rounds of air strikes in response to their targeting of international shipping — struck a US-owned bulk cargo carrier in the wake of the designation announcement, and vowed to continue attacks they say are in support of Palestinians in Gaza.

US forces "conducted strikes on 14 Iran-backed Houthi missiles that were loaded to be fired in Houthi-controlled areas in Yemen," Central Command (CENTCOM) said in a statement.

"These missiles on launch rails presented an imminent threat to merchant vessels and US Navy ships in the region and could have been fired at any time, prompting US forces to exercise their inherent right and obligation to defend themselves," CENTCOM said.

Hani Kayed, a 44-year-old resident of the Yemeni port city of Hodeida, told the media he heard an explosion at around 2:16 am (2316 GMT Wednesday) in the east of the city near the airport.

Hodeida and the city of Taez were among the targets of the first round of US and UK strikes in Yemen last week. Houthi media outlet Al-Masirah TV said both were hit again in the latest strikes, along with three other areas.

Tribal supporters of Yemen's Houthis wave a Palestinian flag and hold up their firearms on Sunday. Reuters

The United States announced earlier on Wednesday that it would return the Huthis to a list of "terrorist" entities.

"The Department of State today is announcing the designation of Ansarallah, commonly referred to as the Houthis, as a Specially Designated Global Terrorist group, effective 30 days from today," Secretary of State Antony Blinken said in a statement.

"During the 30-day implementation delay, the US government will conduct robust outreach to stakeholders, aid providers, and partners who are crucial to facilitating humanitarian assistance and the commercial import of critical commodities in Yemen," he said.

US-owned ship targeted

US National Security Advisor Jake Sullivan said the designation "is an important tool to impede terrorist funding to the Houthis, further restrict their access to financial markets, and hold them accountable for their actions."

"If the Houthis cease their attacks in the Red Sea and Gulf of Aden, the United States will immediately reevaluate this designation," Sullivan said in a statement.

The Houthis, however, said they would not call off their strikes.

US National Security Advisor Jake Sullivan speaks during a meeting.

"We will not give up targeting Israeli ships or ships heading towards ports in occupied Palestine... in support of the Palestinian people," the group's spokesman Mohammed Abdelsalam told Al Jazeera TV, adding that they would respond to new strikes on Yemen by the United States or Britain.

While the Houthis say they have been attacking Israeli-linked vessels, Washington says dozens of countries have connections to the ships that have been targeted in the vital waterway.

The rebels have also declared American and British interests "legitimate targets," and Houthi military spokesman Yahya Saree said in televised remarks that they targeted a US vessel called the Genco Picardy in the Gulf of Aden.

CENTCOM confirmed the vessel had been targeted on Wednesday, saying a drone was launched from Houthi-controlled territory in Yemen and hit the US-owned and -operated, Marshall Islands-flagged bulk carrier ship.

Agence France-Presse