The US military directly targeted Yemen's Huthi rebels for the first time on Thursday, hitting radar sites controlled by the insurgents after US warships came under missile attacks twice in four days.

Defence officials in Washington said five Tomahawk cruise missiles destroyed three mobile radar sites in Huthi-held territory on Yemen's Red Sea coast and that the Pentagon was preparing for possible new retaliatory strikes.

The Shiite Huthi rebels have denied firing missiles at US warships in the Red Sea on Sunday and Wednesday that fell short of their targets.

The United States is providing logistical support to a Saudi-led coalition that has been battling the rebels since last year, but Thursday's bombing marked the first time Washington has taken direct action against the Huthis.

Following the US strikes, Iranian media reported that Tehran which backs the Huthis had dispatched two warships to the Gulf of Aden on October 5, tasked with "protecting commercial ships and oil tankers" from pirates.

The Pentagon said the cruise missile strikes -- authorised by President Barack Obama -- were launched by the destroyer USS Nitze.

"Initial assessments show the sites were destroyed," Pentagon press secretary Peter Cook said in a statement.

The strikes "targeted radar sites involved in the recent missile launches threatening USS Mason and other vessels operating in international waters in the Red Sea and the Bab al-Mandab (Strait)," he said.

"The United States will respond to any further threat to our ships and commercial traffic, as appropriate, and will continue to maintain our freedom of navigation in the Red Sea, the Bab al-Mandab, and elsewhere around the world," Cook added.

The US strikes did not target Huthi missiles and, though the radars' destruction makes it harder to aim the weapons, a senior defence official warned the rebels could still use small spotter boats or even online ship-tracking websites to find new targets.

"They do need to knock it off. We will not hesitate" to launch new retaliatory attacks, said the official, speaking to AFP on condition of anonymity.

- Attack claims 'baseless': rebels -

The Huthis said the accusations they had fired on US warships were "baseless".

"Such claims aim to create false justifications to step up attacks and to cover up for the continuous crimes committed by the aggression against the Yemeni people," the rebel-controlled Saba news agency quoted a military official allied with the insurgents as saying.

The Saudi-led coalition launched its campaign in March last year, after the rebels seized control of large parts of Yemen including Sanaa, forcing the government of President Abedrabbo Mansour Hadi to flee.

The coalition has since carried out hundreds of air strikes and provided ground troops to support Hadi's forces, but it has failed to dislodge the rebels -- who are allied with forces loyal to ex-president Ali Abdallah Saleh -- from key areas.

The US military provides intelligence and refuelling for coalition aircraft. It also supplies advanced munitions and logistics support to the effort, and is Saudi Arabia's biggest arms supplier.

The campaign has faced increasing international criticism over civilian deaths, with critics calling on Washington to end its support for the coalition.

After a coalition air strike on a funeral in Sanaa on Saturday killed more than 140 people, the US administration announced an "immediate review" of its cooperation.

- 'Unlawful' funeral strike -

Human Rights Watch, which has regularly criticised the Saudi intervention, said the funeral strike needed to be investigated as "an apparent war crime".

"While military personnel and civilian officials involved in the war effort were attending the ceremony, the clear presence of several hundred civilians strongly suggests that the attack was unlawfully disproportionate," it said.

Francois Heisbourg, an analyst at the Paris-based Foundation for Strategic Research, said it was unlikely Washington was stepping up its participation in the coalition.

"They reacted in a limited manner and it should stop there," he said.

Soufan Group, a US-based security consultancy firm, said the Huthis "represent an increasingly significant instrument of Iran's regional strategy".

"To counter Saudi Arabia, Iran is supplying the Huthis with capabilities similar to those it provides to its most important regional ally," the Lebanese Shiite militant group Hezbollah, it said.

Yemen's conflict has killed more than 6,800 people, wounded more than 35,000 and displaced at least three million since the coalition launched military operations, according to the United Nations.

As well as supporting the coalition, Washington has for years carried out drone attacks against jihadists in Yemen, regularly killing members of the local branch of Al-Qaeda.