Four people have been confirmed dead and 284 are still missing after the South Korean ferry "Sewol" sank off the southern coast near Jindo, south of Seoul on Wednesday.
Yonhap News Agency reported that dozens of ships and helicopters are searching for nearly 300 missing people who were traveling from Incheon in the north-west to the southern resort island of Jeju. Currently 174, people have been rescued. Reports noted that there were 459 people on board, including 325 students and 15 teachers from Seoul's Ansan Danwon High School.
David Gallo, director of special projects at the Woods Hole Oceanographic Institution, told CNN that rescue workers are facing great challenges, going up against darkness, cold and swift currents, among other obstacles.
"It's just an absolutely, positively horrific situation. It's nightmarish," Gallo said.
What caused the ferry to sink is not yet known, though reports noted that "Sewol" sank within two hours after sending out its first distress call shortly before 9 a.m. local time.
South Korean President Park Geun-hye said she feels "truly devastated" after visiting the central disaster management office in Seoul.
"There is not much time left before sunset. Please make the best efforts to rescue even one more person," she urged.
Two of the confirmed dead are a 27-year-old female crew member and a high school student, though their names were not yet released. A third person who died is believed to be a male student whose body was found later.
One survivor, identified as 57-year-old Yoo, said he heard a bang before the ship started sinking.
"There was a bang and then the ship suddenly tilted over," the survivor said. "Downstairs were restaurants, shops and entertainment rooms, and those who were there are feared to have failed to escape."
BBC News' Transport correspondent, Richard Westcott, noted that domestic ferries across certain parts of Asia have a bad safety record, though mostly in countries like Philippines and Indonesia where overcrowding is a problem.
"This particular ferry was built in Japan, who make some of the best ships in the world. It wasn't anywhere near full and it was traveling a well-worn route in reasonably calm seas. The speed with which it flipped over and sank is a major concern," Westcott said.
"Passengers described a loud thud, which might suggest it hit something hard. It doesn't matter how well built the ship is, collisions can sink vessels very quickly," he added.
"Survivors have also complained that they were told to stay put even though the ferry was tipping over. That's not as surprising as you might think. The basic assumption is always that the ship is the best lifeboat. That it's normally safer to stay on board than brave the water."