The US has launched its biggest military operation in the Middle East against Yemen's Iran-aligned Houthis since Donald Trump took office.
The strike was launched over the group's attacks against Red Sea shipping, killing at least 53 people at the start of a campaign that could last for weeks according to a US official.
Mr Trump warned Iran, the Houthis' main backer, that it needed to immediately halt support for the group.
He said if Iran threatened the United States, "America will hold you fully accountable and, we won't be nice about it!"
A history of the US and the Houthis
The Houthis have waded into the Gaza conflict with attacks on shipping routes in the Red Sea as a show of support for the Palestinians and Hamas.
They have also claimed attacks on ships they say are linked to Israel in the Gulf of Aden, the Arabian Sea, the Indian Ocean and the Mediterranean Sea.
The Houthi attacks have disrupted global shipping, forcing firms to re-route to longer and more expensive journeys around southern Africa.
While the militia has said it attacked only vessels with links to Israel, the United States and Britain, shipping industry sources say all ships were at risk.
The US and Britain had retaliated with air strikes against the Houthis as part of international efforts to restore the free flow of trade along a key route between Europe and Asia that accounts for about 15 per cent of the world's shipping traffic.
A period of relative calm started in January alongside the Gaza ceasefire but the group then warned in March it would resume its naval operations if Israel did not lift a blockage of aid into Gaza.
Houthis 'unlikely' to respond in the way Trump wants, expert says
In response to Trump's attacks, the top commander of Iran's Revolutionary Guards, Hossein Salami, said the Houthis were independent and took their own strategic and operational decisions.
Dr Sarah Phillip said the attack from Mr Trump was an attempt to "squeeze" the Houthis, but suspected they were "unlikely" to respond in the way in which Mr Trump would be hoping and she did not expect the group's behaviour to change.
"This is an organization that have been fighting since 2004 … they are very accustomed to taking hits like this," she said.
"They are very good at moving their equipment around, they are good at sustaining causalities, and they are also fairly unconcerned about sustaining civilian causalities."
United Nations Secretary-General António Guterres on Sunday called for "utmost restraint and a cessation of all military activities" in Yemen.
He also warned new escalation could "fuel cycles of retaliation that may further destabilize Yemen and the region, and pose grave risks to the already dire humanitarian situation in the country," his spokesperson said in a statement.
The unfolding strikes represent the biggest US military operation in the Middle East since US President Donald Trump took office in January.
US Secretary of State Marco Rubio told CBS News there was "no way the Houthis would have the ability to do this kind of thing unless they had support from Iran".
"And so this was a message to Iran: don't keep supporting them, because then you will also be responsible for what they are doing in attacking navy ships and attacking global shipping," he said.