South Korea's coast guard says two people are dead and at least 290 missing after a passenger boat sank off the country's southern coast Wednesday.
The ferry was carrying more than 470 people, mostly students, to the resort island of Jeju when it sank in what appeared to be calm conditions.
Officials initially announced only around 100 people were unaccounted for. But they later drastically increased that number for unknown reasons.
Many could be trapped inside the almost completely submerged ship, though officials say some missing passengers may have been rescued by nearby commercial vessels.
South Korean Vice Minister of Security and Public Information Lee Gyeong-og said a massive rescue effort is under way.
"Eighteen helicopters were mobilized for the rescue work with (a) firefighting team, coast guard, navy, and police forces. Thirty-four vessels from the coast guard and 25 from the navy were mobilized, as well.
A team of South Korean navy SEALS is also participating in the rescue.
The U.S. Navy said it is sending an amphibious assault ship, the USS Bonhome Richard, to join the search and rescue operation.
Most of the passengers were high school students headed to Jeju island on a school outing.
The students attend a school in Ansan, near Seoul. Classes were dismissed Wednesday as anxious parents gathered, waiting for news about their children.
Park Sung-ho is a father of one of the students.
"I can't even describe how I feel now. I was super nervous. Even specially trained soldiers will have a hard time if they get stranded at sea. Then what a shock it must be to those kids? I'm sure it must have felt devastating. It's even worse for the parents."
The ferry departed late Tuesday from the western port of Incheon. It sank about 20 kilometers off the island of Byungpoong.
The reason for the incident is not clear. Some passengers reported hearing a loud noise before the ferry started sinking.
Heavy fog was reported in the area on Tuesday evening, but it is not known whether this contributed to the sinking.