Fresh strikes targeted Yemen's Houthis, security sources and the US military said, after the Iran-backed rebels warned of further attacks on Red Sea shipping.
The strikes came a day after US and British forces hit scores of targets across the country, heightening fears that Israel's war with Palestinian militant group Hamas could engulf the wider region.
Violence involving Iran-aligned groups in Yemen, Lebanon, Iraq and Syria has surged since the war in Gaza began in early October.
The Houthis, who say they are acting in solidarity with Gaza, have carried out a growing number of missile and drone attacks on what they deem Israeli-linked shipping in the key Red Sea international trade route.
Around 12% of global trade normally passes through the Bab al-Mandeb Strait, the Red Sea entrance between southwest Yemen and Djibouti.
But since mid-November the rebel attacks have affected trade flows when supply strains are already putting upward pressure on inflation globally.
The Houthi operations have followed Hamas's unprecedented 7 October attack on Israel which sparked the war still raging in besieged Gaza.
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US Central Command said its forces attacked a Houthi radar site early this morning as "a follow-on action" related to the previous day's strikes.
The Houthis' official media earlier said Al-Dailami airbase in Yemen's rebel-held capital Sanaa had been struck in the latest bombardment.
Later, a Houthi-allied military source told AFP that a site on the outskirts of the Red Sea port city of Hodeida which the rebels used to launch a rocket was hit.
A police source confirmed the latest strike.
UN chief Antonio Guterres's special envoy for Yemen urged "all involved" to avoid actions that would endanger maritime trade and "fuel regional tensions at this critical time".
The UN's Yemen envoy Hans Grundberg noted "with serious concern" the impact of the "increasingly precarious regional context" on Yemen and called for diplomacy to be prioritised.
Britain, the United States and eight allies said strikes on Friday aimed to "de-escalate tensions", but the Houthis vowed to continue their attacks.
Analysts said the Western strikes are unlikely to stop the militants.
They will "diminish but not end the Houthi threat to shipping", said Jon Alterman, director of the Middle East Program at the Centre for Strategic and International Studies.
The Houthis withstood thousands of air raids while battling a Saudi-led coalition for more than seven years and fought six wars against Yemen's government between 2004 and 2010.
"All American-British interests have become legitimate targets" following the strikes, the rebels' Supreme Political Council said.
Hussein al-Ezzi, the Houthis' deputy foreign minister, said the United States and Britain must "prepare to pay a heavy price".
The rebels have controlled much of Yemen since a civil war erupted in 2014 and are part of an Iran-aligned "axis of resistance" against Israel and its allies.
Washington last month announced a maritime security initiative, Operation Prosperity Guardian, to protect maritime traffic in the area. But the Houthis kept up attacks despite several warnings.
The United Nations Security Council held an emergency meeting on the strikes on Friday, days after adopting a resolution demanding the Houthis immediately stop their attacks.
US Ambassador Linda Thomas-Greenfield warned no ship was safe from the Houthi threat in the Red Sea.
Russian Ambassador Vassili Nebenzia denounced the "blatant armed aggression" against the entire Yemeni population.
With the strikes on Friday, the United States and Britain targeted nearly 30 locations using more than 150 munitions, US General Douglas Sims said, updating earlier figures.
Houthi military spokesman Yahya Saree said the raids killed five people and wounded six rebels.
"When I heard the first blast, I was terrified, I thought it was a dream," Hodeida resident Manal Faqirah told AFP, saying she was awakened from her sleep by the strikes.
"When the second blast came, I knew this was a strike, this was war," the 36-year-old added.
Mr Biden called the strikes a successful "defensive action" after the "unprecedented" Red Sea attacks and said he would act again if the Houthis continued their "outrageous behaviour".
But Nasser Kanani, spokesman for Iran's foreign ministry, said the Western strikes would fuel "insecurity and instability in the region" while "diverting" attention from Gaza.
US National Security Council spokesman John Kirby told MSNBC there was "no reason" for an escalation and said Washington was not seeking conflict with Iran.
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Middle Eastern leaders voiced concern, with Saudi Arabia calling for "self-restraint and avoiding escalation".
The kingdom is trying to extricate itself from its nine-year war with the Houthis, though fighting has largely been on hold since a truce in early 2022.
Hamas said it would hold Britain and the United States "responsible for the repercussions on regional security".
However, Fabian Hinz, a research fellow at the International Institute for Strategic Studies, discounted the risk of escalation, "as big players like Iran are keen on avoiding a regional war".
Oil prices rose 4% Friday on fears of an escalation before falling back.
Denmark's Torm on Friday became the latest tanker firm to halt transit through the Red Sea.
Hundreds of thousands of people, some carrying Kalashnikov assault rifles, gathered in Yemen's capital Sanaa on Friday to protest, many waving Yemeni and Palestinian flags, an AFP journalist reported.
