The UN Secretary-General Ban Ki-moon on Wednesday sent a letter to President Park Geun-hye of the Republic of Korea (ROK) to express his deep sadness for the shipwreck off the country's southwestern coast, which left at least four dead and 284 missing.

Ban extended his sincere condolences to the families of the victims and also sent his deep sympathies to the government and people of ROK, UN spokesperson Stephane Dujarric said at a regular briefing.

The passenger ship "SEWOL", with 462 people aboard, mostly high school students, sank in waters near Jindo island at around 11:30 a.m. local time Wednesday. The ship, which departed from ROK's western port city of Incheon Tuesday night, was heading for the southern resort island of Jeju.

As of 9 p.m., local time (0000 GMT), 174 people were confirmed rescued. The number of deaths was feared to surge as hundreds of people are still missing in the icy water.

Around 180 divers were sent to the scene for search and rescue operations during the night time. Most of those missing were estimated to be trapped inside the sunken vessel, said ROK Coast Guard.