JAKARTA - Twenty-one people were killed and another 232 were plucked to safety after a ferry sank in heavy seas off Indonesia's Sumatra island on Sunday, an official said.

"So far 21 people have died and 232 are safe," said Nurdin Basirun, the district chief on Karimun island where the survivors have been brought after the ferry went down around 10:00 am (0300 GMT).

He said the search was ongoing despite rain and strong swells and the final toll could be higher. The actual number of passengers aboard the vessel was unknown and many might still be missing.

"We worry the number killed will increase. We're still in the process of searching and rescuing," he told AFP.

Sea transport director-general Sunaryo earlier quoted Basirun as saying 292 people had been rescued and four killed, but the Karimun chief said this information was inaccurate.

The 147-tonne Dumai Express was sailing from Batam island to Pekanbaru when it went down in high seas off Karimun, which is near Singapore in the north of the Indonesian archipelago, police said.

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

The ship's capacity was only 273 passengers and an investigation is underway to determine if overloading contributed to the accident, Sunaryo said.

"If it was overloaded that's against the rules and we won't tolerate that," he 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."

Overloading is a common practice on Indonesian ferries, leading to regular disasters 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.

Indonesian Navy spokesman Iskandar Sitompul said the vessel sank after being hit by waves as high as three meters (10 feet).

"We're not sure if anyone is trapped in the ferry. Those who have been rescued are traumatized," he said, adding that navy ships were helping with the search and rescue effort.

Officials were unable to describe precisely how the Dumai Express sank, amid reports that it capsized.

"We don't know how exactly the ship went down or how fast it sank, but the waves were very big and it was a very dangerous situation," transport ministry spokesman Bambang Ervan said.

He admitted that it was 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. We're still investigating the cause of the sinking," he said.

Another ferry travelling from Dumai to Moro island, near to where the Dumai Express sank, ran aground at around 2:00 pm but all 270 people on board were safe, Ervan said.

Indonesia's 234 million people are spread across 17,000 islands and are heavily dependent on a network of ships and boats, which have a poor safety record.

Up to 335 people were killed when a heavily overloaded ferry sank off Sulawesi island in January. In December 2006 a ferry went down in a storm off the coast of Java, killing more than 500 people.