And Washington followed up by re-listing the Iranian-backed Houthi militants as a "terrorist" group, subjecting the group to harsh sanctions aimed at cutting off funding and weapons the Houthis have used to attack or hijack ships in vital Red Sea shipping lanes.
Here's White House National Security Council spokesperson John Kirby:
"We took this action because of their continued, reckless, indiscriminate attacks on ship transiting the Red Sea and the Gulf of Aden. These attacks are a clear example of terrorism, violation of international law, a major threat to innocent lives and international commerce."
A Houthi official on Wednesday called the U.S. designation "blackmail," and condemned what he called American and British "aggression" against Yemen.
Attacks by the Iran-allied Houthi militia on ships in the region since November have slowed trade and alarmed major powers.
The Houthis say they are acting in solidarity with Palestinians.
The attacks target a route that accounts for about 15% of the world's shipping traffic and acts as a vital conduit between Europe and Asia.
Speaking at the World Economic Forum in Davos, Switzerland on Wednesday, the Iranian foreign minister said safety of navigation in the Red Sea was tied to the situation in Gaza.
Kirby dismissed that claim.
"The whole argument that this is about the war in Gaza, they're just driving a stake through a straw man. There's nothing there."
The involvement of the American military has further raised fears of a wider regional conflagration as Iran fired rockets into neighboring Iraq and Pakistan, which Tehran said were aimed at Israeli and other other terrorist targets.
"And we have moved additional military resources at the president's order into the region to make sure we can protect those interests."
Kirby said the U.S. would act to defend its interests.
"And, if you doubt it, take a look at what happened just a few nights ago."
