Yemen’s Houthis threatened retaliation and tens of thousands of people took to the streets of the country’s capital after Thursday’s late-night bombing by the US and UK of dozens of sites in rebel-held territory. Five people were killed and six injured in the overnight offensive that the Houthis said on Friday had targeted 73 sites in the capital Sana’a, around the port city of Hodeidah and three other regions. The UK and US defended the attacks, which marked a major escalation of the crisis in the Middle East triggered by the Israel-Gaza war, but leaders in the region condemned them for inflaming tensions in an already volatile climate. A vast crowd of people waving Yemeni and Palestinian flags and chanting “death to America, death to Israel,” gathered in Sana’a’s central square to protest against the western bombing, and other protests took place in at least two other major cities in Houthi-controlled areas of the country. Yahya Sare’e, a spokesperson for the Houthi military, accused “the American-British enemy” of launching brutal aggression “as part of its support for the continuation of Israeli crime in Gaza”. The intervention “will not go unanswered and unpunished”, he said. Reports emerged late on Friday night from the UK’s Maritime Trade Operation of a missile being fired at an unnamed vessel 90 nautical miles south-east of Aden in Yemen. It landed 400 to 500m short of the ship and no damage was reported, an incident unlikely to prompt a further military response. A significant retaliation against international shipping or interests, however, could prompt another round of US-led bombing, although western defence sources said they were monitoring the Houthi response closely and that no further strikes were planned for now. In the small hours of Friday morning, the US said it had struck 60 military targets in 16 locations while Britain’s Royal Air Force bombed two, a sweeping bombing campaign intended to halt a spate of Houthi attacks on shipping in the southern Red Sea that has disrupted world trade and threatened to push up inflation. In a letter to congress, president Joe Biden said that he ordered the strikes to protect US personnel, and disrupt the Houthis’ ability to carry out future attacks in the Red Sea, warning: “The United States stands ready to take further action, as necessary and appropriate, to address further threats or attacks.” The UK prime minister, Rishi Sunak, said during a visit to Kyiv that he believed the attacks represented “necessary, proportionate and targeted action” which had successfully degraded the Houthis’ military capability. The bombing was justified because the Houthis were deterring merchant shipping from using the busy international waterway, Sunak said. “People can’t act like this with impunity and that’s why together with allies we’ve decided to take this action,” he said. The White House national security spokesperson, John Kirby, told MSNBC that the airstrikes had sought to target the Houthis’ ability to store, launch and guide missiles and drones, and insisted the bombing of Yemen was not escalatory. “We’re not looking for conflict with Iran. We’re not looking to escalate and there’s no reason for it to escalate beyond what happened over the last few days,” he said, although he called on Iran to stop supporting the Houthis by helping to supply them with the missiles and drones used in the Red Sea. Experts warned, however, that the western bombing, though at the high end of the range expected, would be unlikely to deter the Houthis from attempting some form of retaliation in turn risking further airstrikes against one of the world’s poorest countries. Fabian Hinz, a Middle East expert at the International Institute for Strategic Studies, said the scale of the US strikes was “clearly not symbolic” but that “if you look at the drones and missiles that the Houthis have been using against international shipping, they are mobile and compact and the Houthis are experts at hiding them”. Before the bombing the Houthi leader, Abdul-Malik al-Houthi, said: “Any American attack will not remain without a response.” Hinz said: “They are talking like they are in a war with the US and they will not want to walk away from that.” Aid agencies said they were alarmed by the prospect of further fighting in a country just emerging from a nine-year civil war and called for de-escalation. An Oxfam spokesperson said: “The humanitarian situation in Yemen remains dire with almost 21 million people in desperate need of food, water and life-saving aid. It is vital that peace is restored and further suffering prevented.” Western military sources believe that any Houthi retaliation would be most likely to be aimed at international shipping, but the prospect of the rebel group trying to escalate further by targeting US and UK military bases around the Arabian peninsular with medium-range missiles is of particular concern. The Houthis, who are backed by Iran, control of the north and west of Yemen. They have conducted 27 attacks against international shipping in the southern Red Sea area since mid-October in what they say is a concerted campaign to target ships with Israeli links in support of Hamas in Gaza. Many of the their attacks, however, have been directed at merchant ships without Israeli links, prompting the US and a coalition of allies to dispatch warships to the Red Sea to protect shipping lanes, see off raiding boats and shoot down missiles and drones when necessary. The White House said Biden had decided to bomb targets in Yemen on Tuesday, shortly after the Houthis attacked a fleet of US and UK warships with 18 drones and three missiles. Once that attack was repelled, the US president directed the country’s military to respond, and Sunak decided to join in soon. Middle Eastern leaders voiced concern that the attacks risked further escalation in the region at a time when Israel’s assault on Hamas in Gaza was ongoing, and tensions on the country’s northern border with Hezbollah group, also backed by Iran, were high. Most vocal was Recep Tayyip Erdoğan, the president of Nato member Turkey. “It is as if they aspire to turn the Red Sea into a bloodbath,” he said, accusing the US and UK of acting disproportionately, and indicating that he expected the Houthis to retaliate by giving “the necessary response in the region”. Saudi Arabia, which is engaged in peace talks with the Houthis, said it was monitoring the situation with great concern. Riyadh said: “The kingdom emphasises the importance of maintaining the security and stability of the Red Sea region, as the freedom of navigation in it is an international demand.” No other country was prepared to participate in the US-UK bombing, although it received help from Australia, Bahrain, Canada and the Netherlands. Denmark, Germany, New Zealand and South Korea signed a statement in support of the action, and France said the Houthis were to blame for the escalation.