Some 50 ships and helicopters are currently racing against time to rescue the 476 passengers aboard a sinking South Korean passenger ferry.
The ferry's passengers included 324 students and 14 teachers from Danwon High School.
South Korea's disaster control agency said 179 people have been rescued so far, including all of the students and teachers.
One person, identified as Park Ji Young, a female employee of the boat operator, had died.
Authorities have yet to determine the cause of the accident. Witnesses however narrated hearing a loud impact. Minutes later they saw ship listing and quickly sinking.
"We heard a big thumping sound and the boat stopped," one of the rescued passengers told the YTN news channel. "The boat is tilting and we have to hold on to something to stay seated."
Sailing from Incheon, a port west of Seoul, to the resort island of Jeju, the 6,825-tonne Sewol ferry managed to send out a distress signal about 20km (12 miles) off the island of Byungpoong at about 09:00 local time (00:00 GMT) on Wednesday morning before it started sinking, according to various news agencies.
Jeju is 60 miles off the south coast of South Korea.
The rescue team includes 34 naval, coastguard and civilian ships, as well as 18 helicopters. Navy divers were also seen on the scene.
Yonhap news agency reported that South Korean President Park Geun-hye had ordered coast guard commandos to search the entire ship to ensure no-one was left behind.
South Korean television pictures showed the Sewol lying on its port side. Afterwards, the ship completely capsized, with only the forward part of its white and blue hull showing above the water.
A part of a South Korean passenger ship (C) that has been sinking, is seen at the sea off Jindo April 16, 2014. A passenger ferry carrying about 470 people was listing badly off South Korea's southwest coast and had sent a distress signal, a Korean coast guard official said on Wednesday, and a rescue mission was underway. REUTERS/South Korean Navy/YonhapREUTERS/South Korean Navy/YonhA South Korean passenger ship "Sewol" is seen in this undated photo released by Yonhap on April 16, 2014. All 338 high school students and teachers on board a South Korean passenger ferry that was reported to be sinking on Wednesday have been rescued, a school official told Reuters. The ferry, identified as the Sewol, was carrying about 470 passengers, including the students and teachers, en route to Jeju island, about 100 km (60 miles) south of the Korean peninsula. It sent a distress signal after it began to list badly. REUTERS/Yonhap REUTERS/YonhapHelicopters are seen above a South Korean passenger ship (C) that has been sinking, in the sea off Jindo April 16, 2014. All 338 high school students and teachers on board a South Korean passenger ferry that was reported to be sinking on Wednesday have been rescued, a school official told Reuters. The ferry, identified as the Sewol, was carrying about 470 passengers, including the students and teachers, en route to Jeju island, about 100 km (60 miles) south of the Korean peninsula. It sent a distress signal after it began to list badly. REUTERS/YonhapREUTERS/YonhapA South Korean maritime police helicopter rescues passengers from a sinking ship in the sea off Jindo, April 16, 2014, in this still image provided by Korea Coast Guard and released by Yonhap. All 338 high school students and teachers on board a South Korean passenger ferry that was reported to be sinking on Wednesday have been rescued, a school official told Reuters. The ferry, identified as the Sewol, was carrying about 470 passengers, including the students and teachers, en route to Jeju island, about 100 km (60 miles) south of the Korean peninsula. It sent a distress signal after it began to list badly. REUTERS/Korea Coast Guard/Yonhap REUTERS/Korea Coast Guard/YonhA passenger is rescued by South Korean maritime policemen from a sinking ship in the sea off Jindo April 16, 2014, in this picture provided by West Regional Headquarters Korea Coast Guard and released by News1. About 160 passengers, including high school students, were plucked to safety on Wednesday in a dramatic rescue from a South Korean passenger ferry "Sewol" sinking with 475 on board, officials said, although at least two people had died. REUTERS/West Regional Headquarters Korea Coast Guard/News1REUTERS/West Regional HeadquarATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH An injured passenger rescued by South Korean maritime policemen from a sinking ship in the sea off Jindo, is treated at a port in Jindo April 16, 2014. About 160 passengers, including high school students, were plucked to safety on Wednesday in a dramatic rescue from a South Korean passenger ferry "Sewol" sinking with 475 on board, officials said, although at least two people had died. REUTERS/Park Cheol-hong/YonhapREUTERS/Park Cheol-hong/Yonhap
