Meanwhile, the coast guard said it was investigating whether the ferry's captain was one of the first ones off the sinking ship.
The first instructions from the captain were for the passengers to put on life jackets and stay put, and it was not until about 30 minutes later that he ordered an evacuation, Oh Yong-seok, a 58-year-old crew member, told The Associated Press. But Oh said he wasn't sure if the captain's order, given to crew members, was actually relayed to passengers on the public address system.
Several survivors also told the AP that they never heard any evacuation order.
The loss of that precious time may have deprived many passengers of the opportunity to escape as the Sewol sank Wednesday, not far from the southern city of Mokpo.
Twenty people, including a female crew member, at least five students and two teachers, were confirmed dead by coast guard officials Thursday night. But the toll was expected to jump amid fears that more than 270 missing passengers - many high school students - were dead. Coast guard officials put the number of survivors Thursday at 179.
Video obtained by AP that was shot by a survivor, truck driver Kim Dong-soo, shows the vessel listing severely as people in life jackets cling to the side of the ship to keep from sliding. A loudspeaker announcement can be heard telling passengers to stay in their quarters.
The increasingly anxious search for the missing was hampered all day Thursday by strong, dangerous currents, rain and bad visibility. Officials said divers would continue trying overnight to enter the ship, hoping for gentler currents.
There were 475 people aboard, including 325 students on a school trip to the tourist island of Jeju in the south of the country. The ferry had traveled overnight from Incheon on the northwestern coast of South Korea and was three hours short of its destination when it began to list. The cause is not yet known.
The 146-meter (480-foot) Sewol now sits - with just part of its keel visible - in waters off Mokpo, about 470 kilometers (290 miles) from Seoul.
Oh, a helmsman on the ferry with 10 years' shipping experience, said that when the crew gathered on the bridge and sent a distress call the ship was already listing more than 5 degrees, the critical angle at which the ship can be brought back to even keel.
At about that time, a third mate reported that the ship could not be righted, and the captain ordered another attempt, which also failed, Oh said. A crew member then tried to reach a lifeboat but tripped because the vessel was tilting, prompting the first mate to suggest to the captain that everyone should evacuate, Oh said.
The captain agreed and ordered an evacuation, but Oh said that amid the confusion and chaos on the bridge he does not recall the message being conveyed on the public address system.
By then it was impossible for crew members to move to passengers' rooms to help them because the ship was tilted at an impossibly acute angle, he said. The delay in evacuation also likely prevented lifeboats from being deployed.
"We couldn't even move one step. The slope was too big," said Oh, who escaped with about a dozen others, including the captain.
At a briefing, Kim Soo-hyun, a senior coast guard official, told reporters that officials were investigating whether the captain got on the first rescue boat, but didn't elaborate.
Passenger Koo Bon-hee, 36, told the AP that many people were trapped inside by windows that were too hard to break. He wanted to escape earlier but didn't because of the announcement that said passengers should stay put.
"The rescue wasn't done well. We were wearing life jackets. We had time," Koo, who was on a business trip to Jeju with a co-worker, said from a hospital bed in Mokpo where he was treated for minor injuries. "If people had jumped into the water ... they could have been rescued. But we were told not to go out."
It is not clear if the captain's actions violated any procedures, and he may have believed at the time that it was still possible to control the vessel, which would have made the order to evacuate unnecessary.
Worried and angry parents of the students gathered at Danwon High School in Ansan, which is near Seoul, while other relatives assembled on Jindo, an island near where the ferry slipped beneath the surface, leaving only the blue-tipped, forward edge of its keel visible.
In Mokpo, relatives of the dead students wailed and sobbed as ambulances drove away with the bodies, headed to Ansan. The families, who spent a mostly sleepless night at the Mokpo hospital, followed the ambulances in their cars. At the school, some desperate relatives lashed out in frustration, screaming threats at journalists. On Jindo island, one woman passed out and was carried to an ambulance.
The family of one of the dead, 24-year-old teacher Choi Hye-jung, spoke about a young woman who loved to boast of how her students would come to her office and give her hugs.
"She was very active and wanted to be a good leader," her father, Choi Jae-kyu, 53, said at Mokpo Jung-Ang Hospital while waiting for the arrival of his daughter's body. Choi's mother, sitting on a bench at the hospital, sobbed quietly with her head on her knee.
While more than 400 rescuers searched nearby waters, coast guard spokesman Kim Jae-in said that in the next two days, three vessels with cranes onboard would arrive to help with the rescue and salvage the ship. Divers were working in shifts in an attempt to get inside the vessel, he said, but strong currents wouldn't allow them to enter.
Kim said that divers planned to pump oxygen into the ship to help any survivors, but first they had to get inside.
The water temperature in the area was about 12 degrees Celsius (54 Fahrenheit), cold enough to cause signs of hypothermia after about 90 minutes of exposure, officials said. The ocean was 37 meters (121 feet) deep in the area.
Kim said coast guard officials were questioning the captain, but declined to provide details or speculate on the cause of sinking.
"I am really sorry and deeply ashamed," a man identified by broadcaster YTN and Yonhap news agency as the captain, 68-year-old Lee Joon-seok, said in brief comments shown on TV, his face hidden beneath a gray hoodie. "I don't know what to say."
Kim Han-sik, president of Chonghaejin Marine Co., the ship's owner, also apologized separately, bowing deeply and saying, "I committed a sin punishable by death. ... I am at a loss for words. I am sorry. I am sorry."
The last major ferry disaster in South Korea was in 1993, when 292 people were killed.
Klug reported from Seoul. Associated Press writers Hyung-jin Kim in Ansan and Jung-yoon Choi in Seoul contributed to this report.