An overloaded ferry has capsized after it hit rocks in a remote river in Malaysia's Borneo, leaving at least 21 people missing, Malaysian police say.

The vessel was believed to be carrying much more than its recommended limit of 74 passengers when it sank in the Rajang River on Tuesday, Bakar Sibau, a district police chief in Sarawak state, said.

"An express boat carrying about 100 people has capsized in a swift-flowing river. I believe there were women and children onboard," he said.

He said the boat was believed to have struck a rock while navigating  downstream from Belaga through one of many sections of rapids on the 560km waterway.

"The boat was overloaded," Sibau said.

Malaysia's Borneo island

Abdul Halim Abdullah, the state government's local administrator for Belaga, told AFP news agency the boat capsized downstream from the town as it was headed towards the coast.

"Around 21 people have not been accounted for," he said.

The boat was ferrying passengers travelling for the upcoming Gawai festival, one of the most important festivals celebrated each year by Sarawak's indigenous Dayak people.

Police rescuers were heading towards the accident site, Bakar said, adding that he had no information on any casualties.

The location of the accident is deep in the wild and sparsely populated interior of Sarawak, the country's largest state and one of two Malaysian states located on Borneo.

The Sun newspaper had earlier quoted another Sarawak official as saying 20 to 30 people may be trapped in the vessel.

"We suspect 20 to 30 passengers may be trapped inside the boat which capsized at about 9:00 am [local time]," said Liwan Lagang, state assemblyman for Belaga and an assistant Sarawak state tourism minister.

Borneo island is divided among three countries: Indonesian territory in the south, two Malaysian states in the north and tiny Brunei on the north coast.

Source: Agencies