South Korean rescuers have fought strong currents and murky waters in their search for 282 people missing after a ferry capsized off the country's southern coast.
Coast guard officials say 20 people are confirmed dead, although the death toll is expected to rise sharply. A total of 179 passengers have been rescued.
It is unclear if anyone is alive inside the boat, which is almost completely submerged in water that is only 12 degrees Celsius.
U.S. President Barack Obama on Thursday sent his condolences to the families of the victims and said he will pay tribute during a visit to that country next week.
South Korea's Minister of Security and Public Administration Kang Byung-kyu said floating cranes will attempt to lift the boat out of the water.
Families of the missing passengers are furious at the government because of what they say is the slow pace of the rescue effort.
Many of them lashed out in frustration at President Park Geun-hye during an impromptu meeting on the island of Jindo, near the crash site.
Ms. Park tried to reassure the distraught families that authorities are doing everything possible in the search and rescue mission.
"I urged those commanding the rescue work to do their best until the last moment, keeping it in mind how much the families have suffered. That was my order."
The 6,825-ton ferry Sewol departed from the port of Incheon on Tuesday night for the island of Jeju, 100 kilometers off the southwest coast.
Authorities have not established the cause of the disaster. But some survivors report hearing a loud impact noise before the vessel rolled onto its side and began sinking.
Many passengers said they were initially told to stay in their seats and not try to escape, a development that outraged many families of those missing.
The Coast Guard says it is investigating reports on whether the ship's captain, Lee Joon-seok, was among the first to abandon the vessel.
Reporter: "Is there anything you would like to say to families of those missing?"
Ferry Captain: "I am so sorry and I am ashamed. I'm at a loss for words."
Kim Young-bung, the head of the Chonghaejin Marine Company that operated the ferry, also apologized for the disaster.
"We deeply apologized to the families and I'm saying again that we're really sorry. Our company will promise that we'll do our best not to lose any more lives. We're sorry."
Among the ferry's 475 passengers were 325 students from a high school near Seoul, traveling to the popular resort island for a four-day field trip and sightseeing.
Heavy fog was reported in the area on Tuesday evening, but it is not known whether that contributed to the sinking.