The search for survivors of the South Korean ferry disaster has been hampered by bad weather, murky water and strong currents.
Emergency services are still searching for about 280 people missing after a ship carrying 475 people sank.
Officials say 179 people have been rescued. Most of the passengers were pupils at the same high school.
South Korea’s president visited the wreck and urged rescuers to “hurry”.
President Park Geun-hye said that time was running out and that every minute and every second was critical.
Eighteen people are confirmed to have died, with dozens more injured.
South Korea’s Yonhap news agency reported earlier that one Russian and two Chinese were among the missing.
Military divers have been fighting high winds and waves to try to access the vessel but were not able to get into any of the cabins, the Chief of the West Regional Headquarters of the South Korean Coastguard, Kim Soo-hyun, said.
At a press conference on Thursday, Kim Soo-hyun said reports that the ferry went off its course were being investigated.
It is not yet clear what caused the ship to list at a severe angle and flip over, leaving only a small part of its hull visible above water, but some experts have suggested the ship may have hit an underwater obstacle.
Passengers’ relatives are also questioning the role of the captain, who is being quizzed by police.
Captain Lee Joon-seok was shown apologizing on television.
“I am really sorry and deeply ashamed. I don’t know what to say,” he said.
It comes amid reports he was one of the first to escape the doomed ship.
Naval and coastguard vessels have been using floodlights and flares as darkness falls to maintain a search now involving more than 500 divers, 171 vessels and 29 aircraft.
Distraught relatives gathered in a gymnasium on nearby Jindo island insisted more should be done, and vented their grief and frustration to anyone who would listen.
The vessel – named Sewol – was travelling from Incheon port, in the north-west, to the southern resort island of Jeju.
Yonhap said the dead included four 17-year-old students and a 25-year-old teacher as well as a 22-year-old female crew member. Identities of the other victims were not immediately known.
The latest figures say 475 people were on board and 278 are still missing. Figures issued by the government have changed several times, prompting criticism.
Search efforts have been concentrated on the ship, which sank in about 100ft of water.
“We carried out underwater searches five times from midnight until early in the morning, but strong currents and the murky water pose tremendous obstacles,” said Kang Byung-kyu, Minister for Security and Public Administration.
Meanwhile, the coastguard chief denied reports that three divers had been swept away and had to be rescued themselves.
Privately, some officials admit it is unlikely the remaining passengers will be found alive.
“Honestly, I think the chances of finding anyone alive are close to zero,” one coastguard official told an AFP journalist on a rescue boat.
The US Navy has sent an amphibious assault ship, the USS Bonhomme Richard, to assist with the search.
Cranes are expected to reach the scene on Saturday.
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