Washington is seeking to reduce the Iran-backed Huthis' capabilities, but the Yemeni rebels are still able to attack despite a week of strikes, and they have vowed that they will keep targeting merchant vessels.

The Huthis began striking Red Sea shipping in November, saying they were hitting Israeli-linked vessels in support of Palestinians in Gaza, and subsequently declared American and British interests to be legitimate targets as well.

"This morning, US forces conducted three successful self-defense strikes against Huthi targets in Yemen," National Security Council spokesman John Kirby told reporters.

"This is the fourth preemptive action that the US military has taken in the past week against Huthi missile launchers that were ready to launch attacks," Kirby said.

"These actions were... done in self-defense, but it also helps make safer international waters for both naval vessels as well as merchant shipping."

The air campaign against the Huthis began last week with American and British strikes on nearly 30 sites in Yemen using more than 150 munitions, while US forces later attacked a Huthi radar site in what was described as "a follow-on action" to the previous strikes.

Since then, American forces have carried out further air raids against missiles that Washington says were ready to launch and posed a threat to both civilian and military vessels.

Washington is also seeking to put diplomatic and financial pressure on the Huthis, re-designating them as a "terrorist" entity after previously dropping that label soon after President Joe Biden took office.

Yemen is just one part of a dangerous and growing crisis in the Middle East, with Iran-backed militants carrying out attacks on US troops in Iraq and Syria, Israel and Hezbollah exchanging cross border fire on a near-daily basis, and the war against Hamas taking a devastating toll on Palestinians in the Gaza Strip.