Some crew are abandoning a Barbados-flagged ship and others are reported missing after it was damaged in an apparent attack off Yemen, maritime security firm Ambrey says.
A US official told Reuters smoke was seen coming off the True Confidence and a lifeboat was seen in the water.
It follows reports of an explosion 100km (62 miles) south-west of Aden.
Yemen's Houthi movement, which has been attacking commercial shipping in the region, have not yet commented.
The Iran-backed group says the attacks are a show of support for the Palestinians in the war between Israel and Hamas in the Gaza Strip.
Before the incident, the True Confidence had been hailed by a group calling itself the "Yemeni navy" and told to change course, Ambrey said.
Nearby vessels then reported a loud bang and a large plume of smoke, according to the United Kingdom Maritime Trade Operations (UKMTO) agency.
Rescue and salvage operations were under way and come of the crew were already in lifeboats, the EU's Maritime Security Centre-Horn of Africa (MSCHOA) mission said, adding that naval ships were providing assistance.
The True Confidence, a bulk carrier, had been sailing to Jeddah in Saudi Arabia from Lianyungang in China, according to tracking data.
Map showing location of reported attack in the Gulf of Aden on 6 March 2024
On Sunday, a Belize-flagged cargo ship, Rubymar, sank in the Red Sea two weeks after hit by missiles fired by Houthis. It was the first ship to have been sunk since the Houthi attacks began in November.
The Rubymar was near the Bab al-Mandab Strait, which connects the Gulf of Aden with the Red Sea, when it was attacked. The crew was rescued and the vessel began slowly taking on water.
It was carrying a cargo of 21 tonnes of ammonium nitrate fertiliser, which the US military said presented an environmental risk in the Red Sea.
US and British forces have responded to the drone and missile attacks on merchant vessels passing through the Red Sea and the Gulf of Aden by targeting Houthi weapons and infrastructure in western Yemen.
