The Iran-backed Houthis have repeatedly targeted vessels in the vital shipping lane with strikes they say are in support of Palestinians in Gaza, where Israel has killed at least 20,000 people, in its two-and-a-half-month war on the strip.

"We've had over 20 nations now sign on to participate" in the coalition, Pentagon spokesman Major General Pat Ryder told journalists.

Ryder said the Houthis are "attacking the economic wellbeing and prosperity of nations around the world," effectively becoming "bandits along the international highway that is the Red Sea."

Coalition forces will "serve as a highway patrol of sorts, patrolling the Red Sea and the Gulf of Aden to respond to -- and assist as necessary -- commercial vessels that are transiting this vital international waterway," he said, calling on the Houthis to cease their attacks.

Israel's war on Gaza has provoked widespread anger in the Middle East and provided an impetus for attacks by armed groups in the region, including the Houthi strikes on Red Sea shipping.

The United States announced the multinational Red Sea coalition on Monday, while the Houthis warned two days later that they would strike back if attacked.

*This story was edited by Ahram Online