SEOUL, April 18 -- A total of 25 people, including 12 students, two teachers and three crew members, have been confirmed dead, with 179 people rescued and 271 still missing after a South Korean ferry capsized, local media reported Friday.

The passenger ferry, the Sewol, which sank in waters off South Korea's southwest coast Wednesday, had 475 people aboard, including 325 high school students and 15 teachers who had been on the way for a four-day field trip.

The ship departed from South Korea's western port city of Incheon Tuesday night for the southern resort island of Jeju.

Among the rescued, 75 were students from the Danwon High School in Ansan, a Seoul suburb. Almost 70 percent of those aboard were from the high school.

Three large salvage ships arrived at the scene Friday morning to pull the sunken ship out of the waters. Hundreds of coast guard and navy divers were working at the scene, and scores of helicopters, airplanes and rescue ships were dispatched to join the search operations.

The death toll was expected to rise as hundreds of passengers were still missing nearly two days after the 6,825-ton passenger ship capsized and sank off Jindo Island, near the southwestern tip of the Korean Peninsula, at around 11:30 a.m. local time Wednesday.

Two Chinese citizens have been confirmed to be aboard the sunken ferry, one male and the other female, who had been on their way to Jeju Island for vacation. The missing couple were migrant workers working in South Korea for years.

The Coast Guard received the first distress call from an estimated student passenger of the ship at about 8:52 a.m. Wednesday, and the ship had remained afloat in the waters for about two and a half hours with its body tilting.

The Coast Guard said that most of the missing were believed to be trapped inside the sunken vessel. The rescue operations had troubles overnight due to rapid currents and low underwater visibility.

The ferry's sinking was believed to have been caused by a sudden turn in direction, which was executed at around 8:48 a.m. Wednesday according to the automatic identification system data offered by the South Korean Oceans and Fisheries Ministry.

Why the captain suddenly made the turn had yet to be known, but the change in direction was believed to have moved some 180 cars and trucks and over 1,100 tons of shipping containers on the deck of the ship to one side, driving the ship to lean to the port side gradually.

The Sewol reportedly tilted first, rolled over on the port side and capsized before being totally submerged in the waters at around 11:30 a.m. Wednesday. It took around two and a half hours until the ship sank.

It was originally believed that the passenger ship might have run aground as some rescued passengers said they heard a thumping sound on the bow before the ship sinks.

The banging sound was estimated to be a noise made when the vehicles and containers were out of their place and crashed each other.

At the first stage, the ferry was believed to have veered off course as it departed some two and a half hours later than scheduled due to a heavy fog, but the oceans ministry said the ship was not far off its intended route.

Other rescued passengers said an announcement was made through the loudspeakers in the vessel warning them not to move as it would be dangerous. It was said to have raised the death toll as many passengers failed to escape from the vessel timely.

The ferry's regular captain who had been on leave was replaced by the substitute surnamed Lee, who the ship's operator Chonghaejin Marine claimed is a veteran with eight years of experience on the Incheon-Jeju Island route.

This is the second accident involving a Chonghaejin Marine vessel in three weeks. Another Chonghaejin ferry hit a 7.93-ton fishing boat on March 28 en route from Incheon to Baengnyeong Island in the West Sea. The 396-ton ship was carrying about 140 passengers and no injuries were reported in the accident.