HONG KONG — Rescuers struggled to reach 18 Ukrainian sailors whose fate was uncertain Monday after they were trapped underwater in their capsized tugboat in Hong Kong for 36 hours amid strong currents, a government spokesman said.

Officials have said the sailors could have found a pocket of air that would enable them to survive. However, divers did not hear any return signal when they knocked on the capsized boat Sunday.

Attempts to move the tugboat to shallower waters to ease rescue efforts have failed so far because of the strong currents, government spokeswoman Heidi Liu said Monday.

She said rescue divers had not been able to reach the sailors and declined comment on their chances of survival.

Hong Kong's Cable TV reported rescuers were waiting for a larger ship to move the capsized tugboat. Liu said she couldn't confirm the report.

'It's (the tugboat) still in the original position...because the underwater current is strong and the salvage operator may not be able to move it,'Liu said.

The tugboat Neftegaz 67 sank and has been lying upside down underwater at a depth of 35 metres since late Saturday, when it collided with Chinese cargo ship Yao Hai in waters northwest of Hong Kong's outlying Lantau island.

The 80-metre-long Ukrainian vessel sank quickly but the Chinese ship suffered only bow damage and stayed afloat, officials said. All 25 crew members on the Chinese vessel were rescued but only seven of the 25 on the Ukrainian ship were found.

The cause of the accident wasn't immediately clear. Officials say weather conditions were reasonable at the time of the accident and neither ship was overloaded.

The Neftegaz 67 was travelling from the southern Chinese city Shenzhen to an oilfield south of Hong Kong.