At least 230 people have been rescued after a ferry sank off Indonesia's coastal island of Sumatra

While an official death toll has not been revealed, police on Sunday said up to 21 people are feared dead.

"The latest data [we have are] 250 people were rescued, including the ferry captain and 12 crew members," Lieutenant Colonel Edwin said.

The 147-tonne Dumai Express was sailing from Batam island to Pekanbaru when it went down around 03:00GMT in heavy rain and huge swells off Karimun island, near Singapore in the north of the Indonesian archipelago, police said.

Iskandar Sitompul, a navy spokesman, said the vessel sank after being hit by waves as high as three metres, but other officials said the waves were towering up to six metres.

"We're not sure if anyone is trapped in the ferry. Those who have been rescued are traumatised," he said.

Edwin said passengers probably jumped off the ferry to save themselves as the ferry was sinking, adding that there were life jackets aboard.

Fishermen were reported among the first rescuers on the scene and pulled many of the survivors to safety.

Officials said the death toll could rise as the number of people aboard the boat was unknown.

Overloaded ferries

The Dumai Express has a capacity 273 passengers and crew, but overcrowding is common on Indonesian ferries.

Bambang Ervan, a transport ministry spokesman, said that it is not uncommon for Indonesian ferries to be overloaded. "In normal conditions ferries can sail with a bit of over-capacity, but in heavier seas it's not good for stability."

Officials said that an investigation is underway to determine if overloading contributed to the accident.

"If it was overloaded that's against the rules and we won't tolerate that," Sunaryo, a sea transport director general, said.

"We will investigate if the ferry was fit to sail and if its documents were complete. We'll also check whether the ship's captain and port master went ahead despite the bad weather or if the weather changed."

Ferry disasters are common in Indonesia despite repeated official promises to tighten and enforce safety regulations.

Corruption is a major problem, with ships selling more tickets than they should and packing ferries with cargo in addition to passengers.