Helicopters are seen above a South Korean passenger ship (C) that has been sinking, in the sea off Jindo April 16, 2014.[Photo/Agencies]

SEOUL/JINDO, South Korea  -- A passenger ship carrying 459 people, mostly high school students, sank off South Korea's southern coast on Wednesday, leaving 291 people missing. At least four people were confirmed dead and 55 injured.

Passengers rescued

S.Korea ferry sinks

More than seven hours after the accident, fears have grown that many of those unaccounted for could be trapped inside the sunken ship and died, though massive rescue and search efforts are still under way.

A second team of divers was to go underwater soon, officials said.

According to the latest government figures, the 6,325-ton Sewol was carrying 462 people, including 325 students from a high school in Ansan, just south of Seoul, when it sent out a distress signal at 8:58 am in waters 20 kilometers off the island of Byeongpoong.

Of them, 174 have been rescued, four died and 284 remain unaccounted for, according to Yonhap News Agency.

The government had earlier announced that 368 people were rescued, but officials later acknowledged there was an error in tallying up figures.

Two of the four dead were a 27-year-old female crew member and a high school student. The crew member was found dead in the ship, while the student died after being rescued. A third death was confirmed later in the day after the body of a male believed to be a student was found. No detailed information was given about the fourth victim.

The accident prompted a massive rescue operation involving about 40 Coast Guard and military vessels and a dozen military aircraft and helicopters. About 200 Navy special forces and divers were also mobilized to search the sunken ship, the defense ministry said

The ship sank at a depth of 30 meters, with the ocean currents flowing at a speed of 8 km per hour.

Additional forces and ships are on the way to the area, the Navy said.

A US Navy amphibious assault ship equipped with two helicopters, which was on its routine patrol mission in the western sea, was moving to the scene to help with the rescue operation, US Forces Korea said.

Television footage showed the ship sinking on its side and rescue workers in orange uniforms trying to help passengers leave the vessel as helicopters were flying overhead. Rescued passengers were wrapped in beige blankets one by one after arriving at a nearby port.

The ill-fated ship completely capsized and sank in two hours.

The cause of the accident was not known, though survivors said they heard a banging noise before the ship suddenly started sinking. Speculation has arisen that the ship might have hit an underwater rock or collided with another vessel.

"There was a bang and then the ship suddenly tilted over," said a survivor identified by his surname Yoo, 57. "Downstairs were restaurants, shops and entertainment rooms, and those who were there are feared to have failed to escape."

The government said the priority is rescue operations.

"We will try to determine the cause of the accident after rescue operations are over," Second Vice Home Affairs Minister Lee Gyeong-og said during a press briefing in Seoul.

Lee said the government will mobilize all available resources to search for the missing.

The ferry set off from the western port of Incheon on Tuesday evening later than scheduled due to dense fog, and was to arrive at the southern resort island of Jeju later on Wednesday. The students were on their way to Jeju for a four-day school trip.

The ship, which plies between Incheon and Jeju twice a week, was built in 1994. It is 146 meter long and 22 meter wide, and has the maximum capacity of carrying 921 people, 180 vehicles and 152 shipping containers at the same time.

South Korean President Park Geun-hye ordered maximum efforts to rescue all of the passengers, stressing that all available Navy, Coast Guard and other vessels nearby should be mobilized to makes sure that not a single life is lost, presidential spokesman Min Kyung-wook said.

In a phone call to Coast Guard chief Kim Suk-kyoon, Park also ordered the mobilization of Coast Guard commandos to search every corner of the vessel to ensure that no one is left behind, the spokesman said.

There were no Chinese passengers on board the vessel, according to the Chinese Embassy in South Korea.

Cargo ship sinks off ROK, 9 die