U.S. President Donald Trump launched large-scale military strikes against Yemen's Iran-aligned Houthis on Saturday over the group's attacks against Red Sea shipping, killing at least 31 people at the start of a campaign expected to last many days.

Trump also 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!"

The top Commander of Iran's Revolutionary Guards reacted on Sunday by saying the Houthis are independent and take their own strategic and operational decisions.

"We warn our enemies that Iran will respond decisively and destructively if they take their threats into action," Hossein Salami told state media.

The unfolding strikes - which one U.S. official told Reuters might continue for weeks - represent the biggest U.S. military operation in the Middle East since Trump took office in January. It came as the United States ramped up sanctions pressure on Tehran while trying to bring it to the negotiating table over its nuclear program.

"To all Houthi terrorists, YOUR TIME IS UP, AND YOUR ATTACKS MUST STOP, STARTING TODAY. IF THEY DON’T, HELL WILL RAIN DOWN UPON YOU LIKE NOTHING YOU HAVE EVER SEEN BEFORE!" Trump posted on his Truth Social platform.

At least 31 were killed and 101 others injured in the U.S. strikes, mostly women and children, Anees al-Asbahi, spokesperson for the Houthi-run health ministry said in an updated toll on Sunday.

The Houthis' political bureau described the attacks as a "war crime."

"Our Yemeni armed forces are fully prepared to respond to escalation with escalation," it said in a statement.

Residents in Sanaa said the strikes hit a building in a Houthi stronghold.

"The explosions were violent and shook the neighborhood like an earthquake. They terrified our women and children," one of the residents, who gave his name as Abdullah Yahia, told Reuters.

Strikes also targeted Houthi military sites in Yemen's southwestern city of Taiz, two witnesses in the area said on Sunday.

Another strike on a power station in the town of Dahyan in Saada led to a power cut, Al-Masirah TV reported early on Sunday. Dahyan is where Abdul Malik al-Houthi, the enigmatic leader of the Houthis, often meets his visitors.

The Houthis, an armed movement that took control of most of Yemen over the past decade, have launched scores of attacks on ships off its coast since November 2023, disrupting global commerce and setting the U.S. military on a costly campaign to intercept missiles and drones that have burned through stocks of U.S. air defenses.

A Pentagon spokesperson said the Houthis have attacked U.S. warships 174 times and commercial vessels 145 times since 2023. The Houthis say the attacks are in solidarity with Palestinians over Israel's war in Gaza with Hamas militants.

The previous U.S. administration of then-President Joe Biden had sought to degrade the Houthis' ability to attack vessels off its coast but limited the U.S. actions.

U.S. officials, speaking on condition of anonymity, say Trump has authorized a more aggressive approach.