The US has carried out a fourth round of strikes on Iranian-backed Houthis in Yemen, Pentagon officials confirmed on Wednesday night.

Some 14 Houthi missiles, which the US says may have been intended for attacks on shipping in the Red Sea, were targeted by American forces.

Centcom - the US Middle East command - said the sites were struck by Tomahawk missiles fired from US Navy vessels.

The strikes come as the US designated the Houthis as "global terrorists".

US National Security Advisor Jake Sullivan said the move, which reversed a decision to remove the designation in the early days of the Biden administration, was in response to the militants' ongoing attacks on commercial shipping in the region.

The Houthis began attacking merchant vessels in November, saying they were responding to Israel's military operation in Gaza. Since then, the group has launched dozens of attacks on commercial tankers passing through the Red Sea, one of the world's busiest shipping lanes.

In response, the US and UK launched a wave of air strikes against dozens of Houthi targets on 11 January. The strikes - supported by Australia, Bahrain, the Netherlands and Canada - began after Houthi forces ignored an ultimatum to cease their attacks in the region.

Wednesday's strikes on Houthi targets followed further attacks on commercial shipping. Hours earlier, the group struck a US owned and operated vessel for the second time this week.

The rebels used a one-way drone to target the M/V Genco Picardy in the Gulf of Aden, Centcom said. The ship suffered some damage, but no crew members were injured in the attack.

On Monday, the Houthis hit another US vessel with a ballistic missile, in what appeared to be the group's first successful attack on an American ship since their campaign began.

US officials said Wednesday's strikes were pre-emptive, and destroyed weapons which were due to be fired imminently by the militants.

"These missiles on launch rails presented an imminent threat to merchant vessels and US Navy ships in the region and could have been fired at any time, prompting US forces to exercise their inherent right and obligation to defend themselves," Centcom said in a statement.

"These strikes, along with other actions we have taken, will degrade the Houthi's capabilities to continue their reckless attacks on international and commercial shipping in the Red Sea, the Bab-el-Mandeb Strait, and the Gulf of Aden," the statement added.

The UK's ministry of defence told the BBC that it was not involved in the latest wave of strikes.

Centcom's commander, Gen Micheal Kurilla, said the US would continue to take action against the Houthis as long as its fighters "continue to endanger international mariners and disrupt the commercial shipping lanes in the Southern Red Sea and adjacent waterways".

Yemen has been devastated by a civil war that escalated in 2015, when the Houthis seized control of large parts of the west of the country from the internationally recognised government and a Saudi-led coalition intervened in an effort to restore its rule.

The fighting has reportedly left more than 160,000 people dead and triggered one of the world's worst humanitarian crises, with two thirds of the population - 21 million people - in need of some form of aid.