Eighteen Ukrainian sailors were trapped underwater Sunday in their capsized tugboat after it collided with a cargo ship in Hong Kong waters, a marine official said.

Hong Kong Director of Marine Dept. Roger Tupper said the sailors were trapped in the ship's hull and could still be alive if they found an air pocket inside the vessel. However, he said rescue divers knocked on the boat and did not hear the sailors signal back.

Divers had made nine attempts to rescue the trapped sailors but were hindered by strong currents and poor visibility, the Hong Kong government said in a statement.

Rescuers were trying to move the 264-foot-long Ukrainian boat — which was upside down at a depth of 115 feet — to a shallower spot to ease rescue efforts, Tupper said.

"We have to get the vessel to shallow water to enable divers to have a better environment to operate," he said at a news conference. "That does take time. That's very unfortunate."

The Ukrainian tugboat Neftegaz, which had been traveling from the southern Chinese city of Shenzhen to an oil field south of Hong Kong, collided with Chinese-registered cargo ship Yao Hai late Saturday off Hong Kong's outlying island of Lantau, Tupper said.

He said six Ukrainians and a Chinese citizen were rescued from the tugboat and efforts were continuing to find the others.

Tupper said that the cause of the accident was not yet known and that weather conditions were "reasonable" when it occurred. Neither vessel was overloaded, he said.

All 25 crew members aboard the Chinese cargo ship were rescued, Tupper said.

The cargo ship suffered some damage but did not sink, the government statement said.