Britain and the US are poised to launch strikes against Houthi military targets in Yemen, as the leader of the rebel group promises to respond to any assault with fresh attacks on shipping in the Red Sea. Western defence sources indicated preparations were intensifying on Thursday in response to a Houthi attack of 21 missiles and drones aimed at US and UK warships on Tuesday night, although its scale and timing remain secret. Asked about potential US strikes against the Houthis in Yemen, the national security spokesperson, John Kirby, said: “I’m not going to telegraph our punches one way or another here. We’re gonna do what we have to do, to counter and defeat these threats that the Houthis keep throwing up on commercial shipping in the Red Sea.” On Wednesday, Antony Blinken, the US secretary of state, said “there will be consequences” following the Houthi attack – while the British defence secretary, Grant Shapps, told journalists to “watch this space”. In response, Abdul-Malik al-Houthi, the leader of the Iran-backed group, which controls the north and west of Yemen, said on Thursday that if struck the Houthis would fight back, suggesting any conflict in the Red Sea would be extended. “Any American attack will not remain without a response. The response will be greater than the attack that was carried out with 20 drones and a number of missiles,” al-Houthi said in a televised address. “We are more determined to target ships linked to Israel, and we will not back down from that,” he added. Rishi Sunak’s cabinet was meeting virtually on Thursday evening amid expectations of UK involvement in imminent military strikes. It is understood that the Labour leader, Keir Starmer, will be briefed after the call. Experts believe the US, UK and other western allies are most likely to target coastal radar and launch sites in a calibrated attempt to halt the three-month spate of Houthi missile and drone attacks in the busy Middle Eastern waterway. But one added any intervention was fraught with risk. Sidharth Kaushal, an expert from the Royal United Services Institute thinktank, said: “It depends what targets are chosen, but the Houthis have said they are not backing down, so there is the risk of sleepwalking into a protracted conflict”. Any strikes – likely to be from air and sea – would have to be enough to act as a deterrent, the analyst added, but a wider bombing campaign added to the risk of civilian casualties and could inflame public opinion in an already volatile Middle East. Aid agencies said they were concerned about the impact of fresh bombing on the Yemeni population, in a country trying to negotiate an end to a nine-year civil war, but one told the Guardian that the Houthi attacks were very popular within Yemen, interpreted as a form of resistance to Israel and the west. The crisis in the southern Red Sea comes at a time of wider conflict in the Middle East. Israel’s bombing campaign against Hamas in Gaza is in its fourth month, while tensions are worsening in the north of the country, where Israeli forces and the Iran-backed Hezbollah in Lebanon have been engaged in increasingly aggressive tit for tat attacks. But a group of western nations, led by the US, increasingly believe that, despite the delicate international situation, there has to be a military response to the Houthis. The group has been gradually stepping up a series of attacks in the southern Red Sea area since mid October in support of Hamas. Using weapons designed in Iran, the Houthis started by targeting merchant shipping, travelling a busy waterway through which an estimated 15% of world seaborne trade flows, and by December prompted major shipping groups such as Maersk and Hapag-Lloyd to route traffic around the Cape of Good Hope, adding at least seven days of travel time and cost. Tuesday night’s attack was deemed to represent an escalation because it directly targeted a group of US and UK warships sent to the Red Sea as protection. Royal Navy sources said HMS Diamond, a destroyer, was among those directly targeted and it was forced to use Sea Viper/Aster missiles and machine guns to defend itself. The US had formed an international naval coalition, Operation Prosperity Guardian, aimed at protecting the waterway, with Australia, Bahrain, Belgium, Canada, Denmark, Germany, Italy, Japan, the Netherlands, Singapore, New Zealand and the UK. The French navy is also operating in the area but is not part of the group. It is led by the US aircraft carrier Dwight D Eisenhower, which has F/A-18 aircraft that could be used to strike targets in Yemen, plus three destroyers, with HMS Diamond acting alongside to protect against aerial targets. Separately, on Thursday Iran seized an US-linked oil tanker in the Gulf of Oman as part of a long-running dispute with the US. News agencies in Iran said it had been seized in response to a court order, and was unrelated to the Houthi attacks, but it added to growing tensions about the safety of commercial shipping in the region. On Wednesday night the UN security council called for an immediate end to attacks by Yemen’s Houthi rebels on shipping in the Red Sea, adopting a resolution that was passed with abstentions from Russia and China. A key provision of Wednesday’s security council resolution, which was sponsored by the US and Japan, noted the right of UN member states, in accordance with international law, “to defend their vessels from attack, including those that undermine navigational rights and freedoms”.