"The top priority of the ongoing rescue operation is to locate the missing persons, both inside the vessel and on the sea surface," the director of the city's marine department, Roger Tupper, told reporters.
The missing were thought to be trapped in the engine room and cabins of a Ukrainian tug, the Naftogaz-67, which collided with a China-registered cargo ship, the Yao-Hai, late Saturday.
The Ukrainian vessel, which was carrying 24 Ukrainian crew and one Chinese sailor, sank quickly after the collision and came to rest on the seabed at a depth of 37 metres (120 feet), Hong Kong officials said.
Six Ukrainians and the Chinese sailor were rescued shortly afterwards, they said.
At Simferopol in Ukraine, the president of the energy company Chornomornaftegaz which owns the ship expressed hopes that the missing sailors would be found alive.
"We have not lost hope of finding the other sailors alive, as long as they are in the interior (of the ship) where there are still pockets of air," said Anatoly Prisiazhnyuk, who said he spoke by telephone with the ship's captain who was one of those who had escaped.
According to the captain, the collision happened because the Chinese vessel refused to give his ship the right of way.
"He had received from the controller permission to pass. The Chinese cargo ship should have let it pass, but it did not do that," said Prisiazhnyuk.
He added that the Naftogaz-67 had been operating since 2002 for the Swiss company Fortranse Ltd in the South China sea between China, Singapore, Malaysia and Indonesia.
It took divers six hours to locate the sunken vessel, which was found several hundred metres (yards) from the location initially given by maritime police, probably because it was moved by a strong current, officials said.
Nine attempts by divers to reach the wreck on Sunday were unsuccessful due to the current, poor visibility and the depth of its position, the government said.
A salvage vessel was dispatched to move the wreck to more shallow water, in the hopes of finding survivors on board.
Ukraine said it would send investigators to the scene on Sunday, Interfax reported from Kiev.
Ukrainian President Viktor Yushchenko called on his government to support the Chinese rescuers and "provide medical aide and all that is needed for the Ukrainian sailors who have been rescued," said his spokeswoman Irina Vannikova in Kiev.