Chinese Consul General Chen Peijie has asked Malaysia to search underwater for the bodies of six people - including five Chinese tourists - missing since a tour boat foundered off Borneo on Saturday night.

Chen, China's top diplomat in Sabah, said the missing may be trapped by marine plants, fishing nets or debris, Xinhua News Agency reported.

"Please check carefully and look carefully for every possibility," Chen asked First Admiral Adam Aziz, Kota Kinabalu director of the Malaysian Maritime Enforcement Agency, Xinhua reported. Kota Kinabalu is the capital of Sabah, one of two states on the Malaysia island of Borneo.

Aziz said Thursday's search operation, involving 22 vessels and other assets, will still focus on the 3,900 square nautical miles searched on Wednesday.

Because the exact position where the boat capsized is not known, their only option is to search a large area, where the water can reach 50 meters deep, he said.

"Our normal divers can only go down to 20 meters, so for 50 meters, we will try to get, maybe, remotely operated vehicles," Aziz said.

The catamaran, carrying 28 Chinese tourists and three crew members, sank in rough seas on its way to the popular tourist island of Mengalum. Three tourists were found dead on Sunday and 22 were rescued after being afloat for more than 30 hours.

Chinese Ambassador to Malaysia Huang Huikang visited some Chinese tourists at Queen Elizabeth Hospital in Kota Kinabalu on Wednesday, where they have been treated for dehydration and severe sunburn.

"We will not give up any gleam of hope and will ... urge the Malaysian side to continue their search efforts and thoroughly investigate the incident so that tourists' safety in Sabah can be ensured," said Huang.

Borneo is a popular Spring Festival destination for Chinese tourists.

News reports have raised questions about the boat's licensing and its operation, including whether it was overloaded and whether safety precautions were taken. Police in Sabah said a thorough investigation will be carried out, and the boat's owner and the two rescued crew members were detained.

Xinhua contributed to this story.