(CNN) -- Rescue boats and helicopters scrambled to pluck passengers, most of them high school students, from a ferry as it slowly sank off the southwest coast of South Korea early Wednesday.
Of the 459 people on board, 164 have been rescued, the security ministry said. Many jumped from the listing ship to the freezing waters of the Yellow Sea.
At least four people -- a female and three males -- were confirmed dead. Another 292 remain unaccounted for, the ministry said.
The rescue operation was still underway Wednesday evening, hours after the ferry first sent out a distress signal.
Authorities could not immediately say what caused the ship to sink. The weather at the time of the incident was clear.
'I wanted to live'
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The ferry, Sewol, was carrying 325 students; 15 teachers; 30 crew members; and 89 other passengers.
The kids were from Seoul's Ansan Danwon High School, and had left from the port city of Incheon, just west of Seoul, for a four-day trip to Jeju. The resort island is considered the Hawaii of Korea.
Around 9 a.m. local time, the ferry sent out its first distress call. It had begun to list.
A rescued student, Lim Hyung Min, told CNN affiliate YTN that he heard a loud bump. Several off his classmates were flung off their feet. Everyone was ordered to don life jackets and jump, he said.
Lim said he jumped into the sea before swimming to a rescue vessel.
"I had to swim a bit to get to the boat to be rescued," he said. "The water was so cold and I wanted to live."
A desperate dash
With the clock ticking, rescue crews dashed desperately to get to passengers.
Soon, the 6,800-ton ferry sank. Only its white and blue hull remained above water.
The rescue operation includes the U.S. Navy ship USS Bonhomme Richard, which was responding to the scene. It dispatched helicopters to the area, the Navy said.
Dive teams have been going in and out of the submerged ship looking for bodies. But low visibility has made their task difficult.
At the Ansan Danwon High School, parents clutched their cell phones in an agonizing wait for a call from their children. Officials posted a list of names. Once a confirmation of a rescue came, they circled that name.
At one point, the school announced that all students had been rescued but soon backtracked, to the parents' wrath.
Passenger Kim Seung Mok said that, despite his efforts and those of others, he couldn't get to several passengers on one of the decks.
"I stayed till the last to rescue people at the hall," Kim told YTN. "But the water was coming in so fast (that) some didn't make it out."
CNN's Euan McKirdy wrote from Hong Kong, while Holly Yan wrote from Atlanta. CNN's Paula Hancocks and journalist Stella Kim in Seoul contributed to this report. CNN's Frances Cha, Madison Park and Judy Kwon also contributed.
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