Officials said the boat carrying Vietnamese tourists sank on Friday after the pump system broke down on the fishing boat that its owner was illegally using to ferry visitors to an island 25km from the mainland.

"Police, navy and army personnel were sent to the scene and managed to rescue 91 people and recover the bodies of 18 others who had drowned," a local official said.

"We dont know exactly how many others are still missing but according to the people on the boat it was carrying around 150 people. We are continuing our search," the official added.

State media, however, reported that nearly 200 people were on board.