Last Updated Apr 15, 2014 11:50 PM EDT
There were media reports that one body had been recovered but that could not be immediately confirmed.
Dozens of rescue boats and helicopters were scrambling to save the passengers. South Korean President Park Geun-hye ordered that all available navy, coast guard and other vessels assist in the rescue effort.The ferry, carrying 476 people including 325 high school students, had set off from the western port of Incheon on Tuesday and was due to arrive at the southern resort island of Jeju later on Wednesday, Yonhap news aaency said.
The vessel was about 12 miles off the island off of the island of Byungpong when it sent out a distress call, saying the boat was listing to one side.
Photographs showed the ship heavily tilted onto its side, partially submerged, as helicopters flew overhead and rescue vessels and a small boat covered with an orange tarp floated nearby.
South Korean media said some passengers who had life jackets had been told to jump into the sea to await rescue.
The students are from a high school in Ansan city near Seoul and they were on their way to the Jeju island for a four-day trip, according to a relief team set up by Gyeonggi Province, which governs the city.