The sailors would have survived 12 hours since the accident late Saturday based on the current water temperature of 17 degrees Celsius (63 Fahrenheit) at the 37-meter (120-feet) depth of the wreck, Roger Tupper, director of Hong Kong's Marine Department, told reporters Monday.
He said rescue divers have continuously knocked on the hull of the ship but the sailors have not signaled back.
"Their chances for survival are very slim," said spokeswoman Zhang Jianwen of China's Guangzhou Salvage Bureau, which is assisting Hong Kong rescue officials.
Tupper said, however, rescuers will continue their efforts until the Ukrainian vessel is retrieved and the bodies are located.
He said bad conditions and the upside down position of the wreck hindered rescue efforts.
"The current is very strong. The visibility is very, very short. It's completely dark. Even in daylight, it's absolutely black down 37 meters (120 feet)," Tupper said.
Zhang said the Guangzhou Salvage Bureau's divers were tying up the Ukrainian tugboat and preparing it for a move from to shallower waters to ease rescue efforts, he said.
Preparation for the move was expected to take several days and a large Chinese salvage ship will be dispatched to Hong Kong from nearby waters Friday, Zhang said. He said the salvage ship would arrive Friday or Saturday.
The tugboat Neftegaz 67 -- which had been detained in Hong Kong in 2003 with safety problems -- sank late Saturday when it collided with Chinese cargo ship Yao Hai in waters northwest of Hong Kong's outlying Lantau island.
The 80-meter-long (264-foot-long) Ukrainian vessel sank quickly but the Chinese ship suffered only bow damage and stayed afloat, officials said. Only seven of the 25 on the Ukrainian ship were found.
The Neftegaz 67 was detained in Hong Kong in September 2003 for safety problems, according to documents from Hong Kong's Marine Department.
The documents said that the ship did not provide "means of escape" or "escape breathing apparatus," and that vessel personnel were not familiar with safety procedures.
It was not immediately clear if those problems were addressed during the detention. Government spokeswoman Heidi Liu said she could not immediately comment.
People who answered the phone at Chernomorneftegaz, the Ukrainian oil and gas exploration company that operates Neftegaz 67, hung up on several calls from The Associated Press.
"We are hoping for the best," Chernomorneftegaz chairman Anatoly Prisyazhnyuk said in comments televised Monday.
Ukraine's ambassador to China, Serhiy Kamyshev, visited the site of the accident Monday. He told reporters afterward that 20 marine experts from Ukraine will arrive in Hong Kong on Tuesday, including some from Chernomorneftegaz, to assist with rescue efforts.
The cause of the Saturday's accident was not immediately clear. Officials say weather conditions were reasonable at the time of the accident and neither ship was overloaded.