By Jonathan Saul

LONDON (Reuters) -A Houthi missile attack killed three seafarers on a Red Sea merchant ship on Wednesday, U.S. Central Command (CENTCOM) said, the first fatalities reported since the Iran-aligned Yemeni group began strikes against shipping in one of the world's busiest trade lanes.

The Houthis claimed responsibility for the attack, which set the Greek-owned, Barbados-flagged ship True Confidence ablaze around 50 nautical miles off the coast of Yemen's port of Aden.

In an earlier message on X responding to the Houthi claim, Britain's embassy wrote: "At least 2 innocent sailors have died. This was the sad but inevitable consequence of the Houthis recklessly firing missiles at international shipping. They must stop."

The Houthis have been attacking ships in the Red Sea since November in what they say is a campaign in solidarity with Palestinians during the war in Gaza.

Britain and the United States have been launching retaliatory strikes against the Houthis, and the confirmation of fatalities could lead to pressure for stronger military action.

CENTCOM said the Houthi strike also injured at least four crew members and caused "significant damage" to the ship. Earlier, a shipping source said four mariners had been severely burned and three were missing after the attack.

The Greek operators of the True Confidence said the vessel was drifting and on fire. They said no information was available about the status of the 20 crew and three armed guards on board, who included 15 Filipinos, four Vietnamese, two Sri Lankans, an Indian and a Nepali national.

A U.S. defence official said smoke was seen coming from the True Confidence. The official, who also declined to be identified, told Reuters a lifeboat had been seen in the water near the ship.

The United Kingdom Maritime Trade Operations (UKMTO) agency said it had received a report of an incident 54 nautical miles southwest of Aden, which lies near the entrance to the Red Sea, adding the vessel had been abandoned by the crew and was "no longer under command".

"Coalition forces are supporting the vessel and the crew," UKMTO said.

Stephen Cotton, general secretary of the International Transport Workers' Federation (ITF), the leading seafarers union, called for urgent action to protect its members.

"We have consistently warned the international community and the maritime industry about the escalating risks faced by seafarers in the Gulf of Aden and Red Sea. Today ... we see those warnings tragically confirmed," Cotton said.

Four days ago, the Rubymar, a UK-owned bulk carrier, became the first ship to sink as a result of a Houthi attack, after floating for two weeks with severe damage from a missile strike. All crew were safely evacuated from that vessel.

The Houthi attacks have disrupted global shipping, forcing firms to re-route to longer and more expensive journeys around southern Africa. The cost of insuring a seven-day voyage through the Red Sea has risen by hundreds of thousands of dollars.

While the militia has said it would attack vessels with links to the United Kingdom, the United States and Israel, shipping industry sources say all ships could be at risk.

The True Confidence is owned by the Liberian-registered company True Confidence Shipping and operated by the Greece-based Third January Maritime, both companies said in their joint statement. They said the ship had no link to the United States.

(Reporting by Jonathan Saul, additional reporting by Enas Alashray and Muhammad Al Gebaly in Cairo and Lisa Baertlein in Los Angeles; Writing by Angus MacSwan and Peter Graff; Editing by Kevin Liffey, Barbara Lewis and Daniel Wallis)