U.S. President Donald Trump said on Monday he would hold Iran responsible for any attacks carried out by the Houthi group that it backs in Yemen, as his administration expanded the biggest U.S. military operation in the Middle East since Trump returned to the White House.

Responding to the Houthi movement's threats to international shipping, the U.S. launched a new wave of airstrikes on Saturday. On Monday, the Red Sea port city of Hodeidah and Al Jawf governorate north of the capital Sanaa were targeted, Houthi-run Al Masirah TV said.

"Every shot fired by the Houthis will be looked upon, from this point forward, as being a shot fired from the weapons and leadership of IRAN, and IRAN will be held responsible, and suffer the consequences, and those consequences will be dire!" Trump said on his Truth Social platform.

The White House said that Trump's message to Iran was to take the United States seriously.

The Pentagon said it had struck over 30 sites so far and would use overwhelming lethal force against the Houthis until the group stopped attacks. The Pentagon's chief spokesperson, Sean Parnell, said the goal was not regime change.

Lieutenant General Alex Grynkewich, director of operations at the Joint Staff, said the latest campaign against the Houthis was different to the one under former President Joe Biden because the range of targets was broader and included senior Houthi drone experts.

Grynkewich said dozens of Houthi members were killed in the strike. The Biden administration is not believed to have targeted senior Houthi leaders.

The Houthi-run health ministry said on Sunday that at least 53 people have been killed in the attacks. Five children and two women were among the victims and 98 have been hurt, it said. Reuters could not independently verify those casualty numbers.

The Houthis, an armed movement that has taken control of the most populous parts of Yemen despite nearly a decade of Saudi-led bombing, have launched scores of attacks on ships off its coast since November 2023, disrupting global commerce.

One U.S. official told Reuters the strikes might continue for weeks. Washington has also ramped up sanctions pressure on Iran while trying to bring it to the negotiating table over its nuclear program.

An Emirati official last week passed on a letter from Trump, who took office in January, proposing nuclear talks with Tehran - a proposal that Supreme Leader Ayatollah Ali Khamenei rejected as "deception" by Washington.

Iranian foreign ministry spokesperson Esmaeil Baghaei said on Monday that Tehran would respond to the letter "after full scrutiny" of it.

The Houthis say their attacks, which have forced companies to re-route ships to longer and more expensive journeys around southern Africa, are in solidarity with Palestinians as Israel strikes Gaza.

The U.S. and its allies characterise them as indiscriminate and a menace to global trade.

Houthi leader Abdul Malik al-Houthi said on Sunday the militants would target U.S. ships in the Red Sea as long as the U.S. continues attacks on Yemen.

Under the direction of al-Houthi, who is in his 40s, the ragtag group has become an army of tens of thousands of fighters and acquired an arsenal of armed drones and ballistic missiles. Saudi Arabia and the West say the arms come from Iran. Tehran denies this.

While Iran champions the Houthis, the Houthis deny being puppets of Tehran, and experts on Yemen say they are motivated primarily by a domestic agenda.

The Houthis' military spokesman, without providing evidence, said in a televised statement early on Monday that the group had launched a second attack against the U.S. aircraft carrier USS Harry S. Truman in the Red Sea.