The officials say the sailors were trapped in the ship's hull and could be alive if they found an air pocket. Rescue divers have knocked on the boat but have not heard the sailors signal back.
Crews are trying to move the tug to shallower waters in hopes of easing rescue efforts.
Six other Ukrainians and a Chinese citizen were rescued from the tugboat. All 25 crew member aboard the Chinese cargo ship were rescued.
The cause of the accident is not yet known.
