Helicopters are seen above a South Korean passenger ship (C) that has been sinking, in the sea off Jindo April 16, 2014.[Photo/Agencies]

CLUES SOUGHT

It was not immediately clear why the Sewol ferry had listed heavily on to its side in apparently calm waters off South Korea's southwest coast, but some survivors spoke of a loud noise prior to the disaster.

A member of the crew of a local government ship involved in the rescue, who said he had spoken to members of the sunken ferry's crew, described the area as free of reefs or rocks and said the cause was likely to be some sort of malfunction on the vessel.

There were reports of the ferry having veered off course, but coordinates of the site of the accident provided by port authorities indicated it was not far off the regular shipping lane.

The ferry sent a distress signal early on Wednesday, the coastguard said, triggering a rescue operation that involved almost 100 coastguard and navy vessels and fishing boats, as well as 18 helicopters.

According to a coastguard official in Jindo, the waters where the ferry capsized have some of the strongest tides off South Korea's coast, meaning divers were prevented from entering the mostly submerged ship for several hours.

The ship has a capacity of about 900 people and an overall length of 146 metres (480 feet). Shipping records show it was built in Japan in 1994.

According to public shipping databases, the registered owner of the ship is Chonghaejin Marine Co Ltd, based in Incheon. Reuters was unable to reach the company by phone.

Earlier, company officials offered an apology over the accident but declined to comment further.

The databases showed that Chonghaejin Marine Co Ltd became the owner of the vessel in October, 2012. ($1 = 1037.6500 Korean Won)