The Western strikes targeting the Huthis' missile and drone capabilities risked inflaming regional tensions, and came despite Biden's previous warnings against any escalation in the Middle East after the October 7 attacks by Hamas on Israel.

The Houthis say they are responding to Israel's military offensive in Gaza, and have vowed retribution for any attack on their assets in Yemen.

Biden, who is seeking reelection in November, said the latest American military intervention in the region was necessary as the Huthi attacks would "endanger freedom of navigation in one of the world's most vital waterways."

"These attacks have endangered US personnel, civilian mariners, and our partners, jeopardized trade, and threatened freedom of navigation," Biden said in a statement. 

"I will not hesitate to direct further measures to protect our people and the free flow of international commerce as necessary," the 81-year-old added.

The strikes followed an "unprecedented" spate of 27 Houthi attacks on ships in the busy international sea route "including the use of anti-ship ballistic missiles for the first time in history," Biden said.

Washington has been intent on acting as part of a coalition as it goes after one of Iran's closest allies, and Biden said Australia, Bahrain, Canada and the Netherlands had provided non-operational support to the US and British strikes.

A joint statement by the US, Britain, Australia, Bahrain, Canada, Denmark, Germany, Netherlands, New Zealand, South Korea said the "aim remains to de-escalate tensions".

"But let our message be clear: we will not hesitate to defend lives and protect the free flow of commerce," it said.

The group's leader Abdel-Malek al-Houthi said on Thursday in a televised speech any US attack on Yemen's Houthis will not go without a response, .

He said any such response would be bigger than the recent one in which its drones and missiles targeted a US ship in the Red Sea. 

Saudi Arabia meanwhile called for restraint and "avoiding escalation" in light of the air strikes launched by the United States and Britain against sites linked to the Houthi movement, the kingdom's foreign ministry said on Friday.

Saudi Arabia, which has in recent months engaged in peace talks with Yemen's Houthis,was closely monitoring the situation with "great concern," the foreign ministry said in a statement. 

"The kingdom emphasizes the importance of maintaining the security and stability of the Red Sea region, as the freedom of navigation in it is an international demand," the ministry added.

'Disrupt and degrade' 

US Defense Secretary Lloyd Austin -- whose job has been on the line after he kept Biden in the dark about his recent hospitalization -- said the strikes aimed to "disrupt and degrade" the Houthis' abilities to target international shipping.

"Today's strikes targeted sites associated with the Huthis' unmanned aerial vehicle, ballistic and cruise missile, and coastal radar and air surveillance capabilities," Austin said in a statement.

Washington set up an international coalition in December -- dubbed Operation Prosperity Guardian -- to protect maritime traffic in the area but the Huthis kept up attacks despite several warnings.

Senior US officials said Biden felt compelled to act after a major Houthi attack on January 9, when US and British forces had to shoot down more than 20 missiles and drones.

"If not for this defensive mission we have no doubt that ships would have been struck, perhaps even sunk, including in one case, a commercial ship full of jet fuel," a Biden administration official.

Biden's statement said the air strikes had "successfully" targeted Houthi sites, but US officials said an assessment of the battle damage was still underway.

However, a senior Biden administration told reporters that the strikes were "significant" and there was "every expectation that it will degrade in a significant way the Houthis' capability."

Whether it would in the long-term deter the Houthis, who remain in control of a swathe of Yemen despite a civil war and a long military operation by a Saudi-led military coalition, remained unclear, the official said.

But the official pointed to Biden's warning of further measures, adding: "This may well may not be the last word on the topic."

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