Almost 300 people were unaccounted for Wednesday after a ferry mainly carrying students on a high school trip capsized and sank off South Korea's coast.
Four deaths were confirmed. One rescued passenger told The Associated Press he believed that many people had been trapped inside the ship when it sank.
Dozens of nearby boats and divers scrambled to rescue passengers who had been aboard the Sewol.
Helicopters airlifted some passengers from the capsized vessel, but it later slipped beneath the waves.
The ferry sent a distress call at about 9 a.m. local time Wednesday (8 p.m. Tuesday ET) after it began leaning to one side, officials said.
Student Lim Hyung-min told broadcaster YTN that he leaped into the ocean and then swam to a nearby rescue boat.
"As the ferry was shaking and tilting, we all tripped and bumped into each another," Lim said, adding that some people were bleeding. Once he jumped, the ocean "was so cold. ... I was hurrying, thinking that I wanted to live."
South Korea's Yonhap news agency reported that passengers had been instructed to jump overboard while wearing life vests.
A South Korean government spokesman told reporters at 5.30 a.m. ET that there had been 459 people on the Sewol, including 325 students, 15 teachers, 89 other passengers and a crew of 30.
Only 164 of those on board were confirmed rescued, and four dead - leaving 291 unaccounted for.
Authorities had earlier announced that most passengers had been rescued but subsequently said there had been an error in tallying the figures.
A government spokesman told reporters one of the dead was a crew member, the second was a male high school student, the third was male - possibly a student. No details were immediately released about the fourth fatality.
The vessel had been traveling to the southern tourist island of Jeju. The trip from Incheon to Jeju is usually about 14 hours, so the ferry was about three hours from its destination the distress call was made.
Some 160 coast guard and navy divers searched for survivors inside the ship's wreckage a few miles from Byeongpung Island, which is not far from the mainland.
Conditions were said to be clear, unlike further north up the coast which had been shrouded in heavy fog that led to the cancellation of many ferry services.
Another witness told YTN there had been a "loud impact and noise" before the vessel started sinking.
Almost 20 helicopters and more than 30 navy and coast guard vessels were at the scene, a government official told Reuters. Navy divers were also deployed, and a U.S. Navy ship that was on routine patrol in the area said it had responded.
The ferry has a capacity of 921 people, 180 vehicles and 152 shipping containers. Shipping records show it was built in Japan in 1994.
Julie Yoo, Jason Cumming and Eric Baculinao of NBC News, David Wyllie of BreakingNews.com, The Associated Press and Reuters contributed to this report.