At least 19 were killed as 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, warning "hell will rain down" on them if they do not stop.

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 unfolding strikes — which one official said would last days and maybe weeks — represent the biggest U.S. military operation in the Middle East since Trump took office in January. It came as the United States ramps up sanctions pressure on Tehran while trying to bring it to the negotiating table over its nuclear program.