The US military said on Sunday its Navy helicopters fired at Iran-backed Houthi rebel boats off Yemen that were attacking a cargo ship, with Yemeni sources reporting 10 rebels killed.

The clash in the Red Sea marked a deadly escalation since the United States set up a multinational naval task force in early December to protect the vital shipping lane against Houthi attacks.

The rebels – who say they are acting in solidarity with Palestinians in the Israel-Hamas war in Gaza – have repeatedly fired drones and missiles at passing ships in the straits through which 12 percent of global trade passes.

US Central Command said the Navy had responded to a distress call from the Maersk Hangzhou, a Singapore-flagged, Denmark-owned and operated container ship that reported coming under attack for a second time in 24 hours while transiting the Red Sea.

The vessel had earlier been targeted with two anti-ship ballistic missiles. One was shot down by the US military and the other hit the Maersk Hangzhou.

The Houthis had then fired on US helicopters, which "returned fire in self-defence", sinking three of four small boats that had come within 20 metres of the ship, according to the CentCom statement.

It said the crews of the three vessels were killed while a fourth boat fled the area.

Maersk suspended the passage of its vessels through the Red Sea strait for 48 hours after the latest of about two dozen attacks by Houthis on international shipping in six weeks. (AFP)