Media playback is unsupported on your device Media caption "Strong currents kept them out of the water and balanced on the hull, tapping for any signs of life", reports Lucy Williamson

Bad weather, murky water and strong currents are hampering the search for survivors of the South Korean ferry disaster.

Emergency services say 25 people are confirmed dead and 271 missing after the ship carrying 475 passengers and crew capsized on Wednesday.

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".

Park Geun-hye said that time was running out and that every minute and every second was critical.

Image copyright AFP Image caption Military divers who attempted to search the ship were reportedly swept out to sea

Image copyright Reuters Image caption Relatives of the missing are enduring an agonising wait for information

South Korea's Yonhap news agency reported earlier that one Russian and two Chinese were among the missing.

'Deeply ashamed'

The small port of Paeng-Mok in Jindo is the centre of the rescue efforts. It is where the coast guard is launching boats that are heading to the submerged ferry more than 20m away. The port is also where many relatives of the missing are waiting for news about their loved ones. When I arrived the police had surrounded a man wearing only trunks and swimming goggles. He was deranged with grief and appeared prepared to do anything to find his child. The police were trying to prevent him from jumping into the sea and launching a futile one-man rescue operation. The government and various aid agencies have set up dozens of stalls to provide a modicum of shelter and comfort for the relatives. There is a first-aid station; a food stall stacked with pot noodles; a shelter were you can charge your phones and computers. In the background, there is a constant hum of generators powering up a bank of satellite trunks. All the South Korean channels are broadcasting round the clock coverage. The eyes of the nation are on this search and rescue operation. At one stage, dozens of relatives surrounded a coastguard official who was giving an update on the rescue operation. During a heated exchange, some of the family members demanded to know why more flares and floodlights had not been used to light up the night sky. Other relatives, wrapped in blankets, sit on plastic chairs waiting for news. But grief is never far from the surface. Standing beside one tent, I heard a woman inside sobbing uncontrollably. Anger and despair

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, Mr Kim said reports that the ferry went off 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.

Capt Lee Joon-seok was shown apologising 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.

But 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.

"Get my child out of that ship! Dead or alive," one distraught father repeatedly shouted to rescue and local government officials.

Strong currents

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.

Media playback is unsupported on your device Media caption Aerial footage shows frantic efforts to rescue passengers as the ship sank

The ship sank in about 30m (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, Mr Kim, 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 Friday.

Image copyright Reuters Image caption Teams used spotlights and flares to light up the scene at night-time

Image copyright AFP Image caption South Korea's president (left, on stage) spoke to relatives of missing passengers, assembled in Jindo port

'Screaming and scrambling'

The ferry sent a distress call at around 09:00 local time (00:00 GMT) on Wednesday, about 20km (12 miles) off the island of Byungpoong. It sank within two hours, reports said.

About 325 of the passengers on board the ship were students from Danwon high school in Ansan, near the capital, Seoul, who were heading on a field trip to Jeju island with about 15 teachers.

Survivors say they heard a loud thud, before the boat began to shake and tilt.

Some of the passengers managed to jump into the ocean, wearing life jackets, and swim to nearby rescue boats and commercial vessels.

But several survivors have said that they were told by crew members not to move.

"We must have waited 30 to 40 minutes after the crew told us to stay put," one unnamed rescued student was quoted as saying by AFP news agency.

"Then everything tilted over and everyone started screaming and scrambling to get out," he said.

Koo Bon-hee, 36, told the Associated Press that the rescue was not "done well". "If people had jumped into the water... they could have been rescued. But we were told not to go out."

Of the people who were rescued, dozens had suffered injuries, officials said.

Image copyright Reuters Image caption Students hold messages of hope for their missing classmates at Danwon High School

Image copyright Reuters Image caption Military and civilian ships, as well as planes and helicopters have been searching for survivors

Some of those trapped managed to send text messages to their relatives.

"Dad, don't worry. I'm wearing a life vest and am with other girls. We're inside the ship, still in the hallway," one girl told her father, AFP news agency reported.

Major maritime accidents in South Korea 1970: Sinking of passenger vessel Namyoung leaves 323 dead

1993: Sinking of passenger vessel Seohae Ferry leaves 292 dead

2007: Sinking of freighter Eastern Bright leaving 14 sailors missing

2009: Sinking of cargo ship Orchid Pia after a collision leaves 16 sailors missing Source: Yonhap

But in a subsequent message she said she could not get out. "The ship is too tilted. The hallway is crowded with so many people."

However, the authorities have cautioned that they do not think that recent text messages purporting to come from the ship, are in fact from people inside.

Kim Young-boong, an official from the company which owns the ferry, has apologised.

Japan's prime minister offered "heartfelt sympathy" to the victims and their families, and his government offered help with the search - a rare moment of detente between the feuding neighbours.

Shinzo Abe, whose strident nationalism has raised tensions between the two countries, said his thoughts were with those caught up in the tragedy.

In a news conference on Thursday, US President Barack Obama expressed his "deepest condolences" to the victims' families and said his country's commitment to South Korea was "unwavering in good times and in bad".

Correspondents say this could turn out to be South Korea's biggest maritime disaster for more than 20 years.