Fire Services commanders said any underwater rescue attempt was difficult and dangerous.

Chow Wing-tak, chief fire officer (Hong Kong Island, Islands and Marine Command), said his divers could stay underwater for only 20 minutes because they faced decompression problems.

'The rescue mission is difficult because the water is turbulent and visibility underwater is poor,' he said. 'The strong current also pushes our divers away from the wreck.'

He added that the current was even stronger than usual on Saturday because it was the middle of the lunar month and a full-moon night.

Director of Fire Services Lo Chun-hung said an attempt would be made to move the tug to shallower water so divers could work on it underwater without facing decompression risks.

Fire Services division commander (Marine and Offshore Islands) Wong Chung-shing would not speculate on the chances of survival of anyone trapped inside.

'When the boat turned upside down, there might have been some air trapped in the cabins,' Mr Wong said. 'The seamen could rely on the air in the cabins for a while, depending on the number of people and the volume of air.'

Crew members were believed to be in cabins and the engine room when the collision between the tug and China-registered cargo ship the Yaohai happened on Saturday night.

Divers had received no response to attempts to communicate with anyone that may be trapped, Mr Wong said.

said.

'We can only send two divers at a time to work underwater,' he said. 'If we send too many divers, their breathing tubes might get tangled. That would be very dangerous for them. Also, the water current is too strong and the entrance into the boat is too narrow.'

Divers were not deployed until after midnight on Saturday night. 'We could not locate the boat with information given to us from the marine police as the strong water current had swept the boat 400 meters from the site where the collision occurred,' Mr Wong said.

'We had to rely on the Marine Department to help us locate the boat.' Government Flying Service assistant manager Lo Yiu-wah said helicopters had made four sea searches for survivors but had found nothing.

'Visibility was only about 2km to 4km. It was very poor,' he said.

The initial search had also been made more difficult because it was at night.

'Two inspections were conducted in the morning, and we could only see some debris floating in the sea.'

Deployed to the scene were 10 marine police launches, six other vessels, five Fire Services vessels and four search helicopters.

Stephen Au Siu-kin, of the Professional Association of Diving Instructors, said the water at the scene was murky and visibility low.

'It is difficult to search for survivors in such circumstances,' he said, adding that the strong current would have made it even harder.

Mr Au, with 15 years' diving experience, said divers could only stay about 20 minutes at that depth. 'It is so dark and murky,' he said.

'Divers could easily be cut by the wreckage and something inside the ship might collapse any time.

'They cannot dive for too long each time or they will suffer from decompression sickness.'