At least 11 crew members have been reported missing after two ships collided off the coast of southern Hong Kong, a police official said.
The incident, which involved a cargo ship and a container vessel, happened in the early hours of 5 May near Po Toi Island, reports said.
The cargo ship is believed to have sunk with its 12-member crew on board. One man was rescued by a fishing boat.
Chinese officials are now conducting a search and rescue mission.
The mission was being co-ordinated by the Guangdong Maritime Rescue Co-ordination Centre and Hong Kong had also sent rescue boats and a helicopter to the area, the South China Morning Post newspaper said.
China was sending more than a dozen ships and at least three helicopters to the area, state-run Xinhua news agency said.
The rescued crew member was in hospital suffering minor injuries, a police official told AFP news agency.
Hong Kong is one of the world's busiest ports and has a relatively strong safety record, says the BBC's Juliana Liu in the territory.
Hundreds of vessels, from commuter ferries to container ships, ply its waters daily, our correspondent adds.
Last year, about 85 people were injured when a high-speed ferry near the island of Hei Ling Chau hit what officials said was an unidentified object.
The ferry, which was travelling from Hong Kong to Macau, had 105 passengers and 10 crew on board.
In 2012, 39 people died when a pleasure boat and a high-speed ferry collided off Lamma Island - considered the worst maritime accident in decades.
The captains of the two vessels were charged with several counts of manslaughter.