Search operations resumed in full scale Thursday for hundreds of passengers missing after their ferry sank off South Korea's southern coast a day earlier in what is believed to be one of the country's deadliest offshore accidents.

Nine out of 475 passengers, mostly high school students, aboard the 6,325-ton ferry Sewol have been confirmed dead so far, while 179 others have been rescued. But the fate of the remaining 287 remains unknown amid growing fears they are trapped inside the sunken ship.

Combing the submerged ship is the top priority in the search, though diving operations have been hampered by poor underwater visibility and strong currents at the scene. A total of 555 Navy, Coast Guard and other divers have been mobilized for the operations, officials said.

The water is very murky as currents stir up mud lying at the bottom of the sea, officials said.

"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, during a press briefing. "We will do our best."

Also mobilized to scour the area are 169 boats and 29 aircraft, Kang said. Two salvage cranes are also on their way to the scene to raise the sunken vessel, with one of them expected to arrive on Friday morning and the other in the evening, the minister said.

The ferry was on its way to the southern resort island of Jeju after from the port of Incheon, west of Seoul, when it sent a distress signal at 8:58 a.m. Wednesday. The circumstance leading to calling for help was not yet known, though survivors said they heard a bang before the vessel started tilting over.

Experts say the vessel could have hit an underwater rock.

The Coast Guard has been questioning the ship's captain and other crew members to determine what went wrong. Coast Guard officials said they found that the ill-fated ship deviated from a government-recommended route.

A total of 325 passengers were students from a high school in Ansan, just south of Seoul. They were on a school trip to Jeju and about 200 of the students remain unaccounted for.

The eight dead included four 17-year-old students and a 25-year-old teacher from the high school, as well as a 22-year-old female crew member. But the identities of the other two were not immediately known.

The government has come under strong fire over its handling of the disaster. It has even been unable to figure out exactly how many people were aboard the ship, and it revised the figure, as well as the numbers of those rescued and missing, many times.

Survivors also blamed the ship's crew, saying they were repeatedly told to stay put where they were, even when the ship began tilting. Had they been told to evacuate earlier, more people would have survived the disaster, they said.

Reports say the ship's captain was one of the first to leave the vessel.

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

The United States expressed its condolences to the families of those killed.

"We extend our deepest condolences to the loved ones of those who lost their lives on board the South Korea ferry, the Sewol," Marie Harf, the State Department's deputy spokeswoman, said at the start of a daily press briefing.

She described the incident as a "terrible tragedy."

The United States is ready to provide any assistance needed in the ongoing search-and-rescue efforts, she said.

On Wednesday, the U.S. military send an amphibious assault ship equipped with two helicopters to the scene to help with the search and rescue operations. The ship was on its routine patrol mission in the western sea.

U.N. Secretary General Ban Ki-moon also expressed condolences and sympathy for the victims. (Yonhap)