The US intends to launch more attacks on Iran-backed groups in the Middle East, the White House national security adviser has said.
Jake Sullivan said: "We intend to take additional strikes, and additional action, to continue to send a clear message that the United States will respond when our forces are attacked, when our people are killed."
He made the comments on NBC’s Meet the Press programme on Sunday, after the US and Britain unleashed attacks against 36 Houthi targets in Yemen on Saturday.
Meanwhile, British Foreign Secretary Lord David Cameron said on Sunday night: "We have issued repeated warnings to the Houthis.
"Their reckless actions are putting innocent lives at risk, threatening the freedom of navigation and destabilising the region. The Houthi attacks must stop."
British Prime Minister Rishi Sunak said the Houthis' "illegal" harassment campaign in the Red Sea is having "economic consequences", as he defended the UK taking part in a third wave of air strikes.
Rishi Sunak said the UK on Saturday had acted in "self-defence" when taking part in joint air strikes with the US to target Houthi sites in Yemen.
He repeated that he "won't hesitate to protect British lives", in a signal that he would be undeterred from continuing to approve further RAF bombing raids if the Houthis do not desist.
But Houthi military spokesman Yahya Saree has said: "These attacks will not discourage Yemeni forces and the nation from maintaining their support for Palestinians in the face of the Zionist occupation and crimes.
"The aggressors' airstrikes will not go unanswered."
Saturday's western attacks came just a day after the US military hit Tehran-backed groups in Iraq and Syria in retaliation for a deadly attack on US troops in Jordan.
The strikes are the latest blows in a conflict that has spread into the Middle East since October 7, when the Iran-backed Palestinian militant group Hamas stormed Israel from the Gaza Strip, igniting war.
Tehran-backed groups declaring support for the Palestinians have entered the fray across the region.
Hezbollah has fired at Israeli targets at the Lebanese-Israeli border, Iraqi militias have fired on US forces in Iraq and Syria and the Houthis have fired on shipping in the Red Sea and at Israel itself.
Iran has so far avoided any direct role in the conflict - even as it backs those groups.
But, on Sunday, it issued a warning to the US over potentially targeting two cargo ships in the Mideast, long suspected of serving as forward operating bases for Iranian commandos.
In a video released by Iran's regular army, a narrator for the first time describes the Behshad and Saviz ships as "floating armories".
The narrator says that the Behshad has aided an Iranian mission to "counteract piracy in the Red Sea and the Gulf of Aden."
However, Iran is not publicly known to have taken part in any of the recent campaigns against rising Somali piracy in the region off the back of the Houthi attacks.
The statement ends with a warning overlaid with a montage of footage of US warships and an American flag."Those engaging in terrorist activities against Behshad or similar vessels jeopardise international maritime routes, security and assume global responsibility for potential future international risks," the video said.
US Defense Secretary Lloyd Austin warned the Houthis after the strikes that "they will continue to bear further consequences if they do not end their illegal attacks on international shipping and naval vessels."
The Pentagon has said it does not want war with Iran and does not believe Tehran wants war either.
Mr Sullivan declined to be drawn on whether his country might attack sites inside Iran, something the US military has been very careful to avoid.
Speaking to CBS' Face the Nation programme moments earlier, he said Friday's strikes were "the beginning, not the end, of our response, and there will be more steps - some seen, some perhaps unseen".
"I would not describe it as some open-ended military campaign," he said.
Saturday's strikes in Yemen hit buried weapons storage facilities, missile systems, launchers and other capabilities the Houthis have used to attack Red Sea shipping, the Pentagon said, adding it targeted 13 locations.
The Houthi military spokesperson Yahya Sarea said the strikes "will not pass without a response and consequences".Another Houthi spokesperson, Mohammed Abdulsalam, indicated the group would not be deterred, saying Yemen's decision to support Gaza would not be affected by any attack.
Residents described being shaken by powerful blasts. "The building I live in shook," said Fatimah, a resident of Houthi-controlled Sanaa, adding that it had been years since she had felt such blasts in a country that has suffered years of war.
The Houthis did not announce any casualties.The Yemen strikes are running parallel to the unfolding US campaign of retaliation over the killing of three American soldiers in a drone strike by Iran-backed militants on an outpost in Jordan.