US President Donald Trump launched large-scale military strikes against Yemen's 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. He said if Iran threatened the United States, "America will hold you fully accountable and, we won't be nice about it!"

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.

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."

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

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.