Attacks by Houthi militia on ships in the region since November have slowed trade between Asia and Europe and alarmed major powers - an escalation of the more than three-month-old conflict between Israel and Palestinian Hamas militants in Gaza.

The Houthis say they are acting in solidarity with Palestinians and have threatened to expand attacks to include US ships in response to the American and British strikes.

In a sign it remains undeterred, the Houthi movement on Wednesday said it made a "direct hit" with missiles on the US Genco Picardy bulk carrier. The attack, which marked the second on a US-linked vessel in the area this week, was confirmed by US shipping operator Genco.

US officials said the "Specially Designated Global Terrorist" listing was aimed at cutting off funding and weapons the Houthis have used to attack or hijack ships.

A Houthi spokesman told Reuters that attacks on ships heading to Israel would continue and the designation would not affect its position.