The United States carried out a fresh strike on Saturday on a Houthi rebel target in Yemen, US Central Command said, after the Iran-backed militants warned of further attacks on ships in the Red Sea.
The strike on a Houthi radar site comes a day after scores of attacks across the country heightened fears that Israel's war on Gaza could engulf the wider region.
The Iran-backed militants' official media earlier said the al-Dailami airbase in Yemen's rebel-held capital of Sanaa had been struck.
The Houthis, who have carried out weeks of attacks on Israel-linked shipping in protest of the Gaza war, warned that US and British interests were "legitimate targets" after the first volley of strikes.
Britain, the United States, and eight allies said strikes carried out on Friday had aimed to "de-escalate tensions", but the Houthis vowed to continue their attacks.
"All American-British interests have become legitimate targets" following the strikes, the rebels' Supreme Political Council said.
Hussein al-Ezzi, the rebels' deputy foreign minister, said the United States and Britain would "have to 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-backed "axis of resistance" against Israel and its allies.
Violence involving Iran-aligned groups in Yemen, Lebanon, Iraq, and Syria has surged since the Gaza war began in early October.
UN chief António Guterres called on all sides "not to escalate" in the interest of regional peace and stability, his spokesman Stéphane Dujarric said.
The UN Security Council held an emergency meeting on the strikes on Friday, days after adopting a resolution demanding the Houthis immediately stop their attacks on ships.
At the meeting, US Ambassador Linda Thomas-Greenfield warned that no ship was safe from the threat posed by Houthi rebels to shipping in the Red Sea.
Russian ambassador Vassili Nebenzia denounced the "blatant armed aggression" against the entire population of the country.
The Houthis have intensified attacks on what they deem Israeli-linked shipping in the Red Sea – through which 12 percent of global maritime trade normally passes – since the Gaza war broke out.
The United States and Britain launched strikes on Friday that targeted nearly 30 locations using more than 150 munitions, US General Douglas Sims said, updating earlier figures, and President Joe Biden said he did not believe there were civilian casualties.
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".
British Prime Minister Rishi Sunak said the Houthis' breach of international law warranted the "strong signal", with his government publishing its legal position justifying the strikes as lawful and "proportionate".
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.
The Houthis fired "at least one" anti-ship ballistic missile in retaliation on Friday that caused no damage, according to Sims.
The United States said it did not seek conflict with Iran, with National Security Council spokesman John Kirby telling MSNBC there was "no reason" for an escalation.
Middle Eastern leaders voiced concern at the violence, with Turkish President Recep Tayyip Erdogan describing the strikes on Yemen as disproportionate and saying: "It is as if they aspire to turn the Red Sea into a bloodbath."
Saudi Arabia said it "is following with great concern the military operations" and called for "self-restraint and avoiding escalation".
The kingdom is trying to extricate itself from a 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".
Oil prices rose four percent on fears of an escalation before falling back.
Major shipping firms have rerouted cargo around the tip of Africa, hitting trade flows at a time when supply strains are putting upward pressure on inflation worldwide.
Since mid-November, the volume of shipping containers transiting through the Red Sea has dropped by 70 percent, according to maritime experts.
Denmark's Torm on Friday became the latest tanker firm to halt transit through the southern Red Sea.
Dryad Global, a maritime security risk group, advised its clients to suspend Red Sea operations for 72 hours, citing the threat of Houthi retaliation.
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 and holding portraits of Houthi leader Abdulmalik al-Houthi, an AFP journalist reported.
"Death to America, death to Israel," they chanted.
In Tehran, hundreds rallied against the United States, Britain, and Israel, burning the three countries' flags outside the UK embassy while voicing support for Gazans and Yemenis, an AFP reporter saw.
In Gaza, Palestinians lauded Houthi support and condemned Britain and the United States.
"No one is standing with us but Yemen," said Fouad al-Ghalaini, one of hundreds of thousands of Palestinians left homeless by Israel's bombardment of Gaza City.