JINDO, South Korea (CNN) -- It began as a routine ocean passage on calm seas. Passengers bound for a resort island were just stirring, some eating breakfast. Suddenly, the ferry began to list. At some point, a loud bang shook the ship.
Suddenly, hundreds of people aboard the ferry Sewol -- many of them teenagers -- faced a terrifying choice: obey commands barked over loudspeakers to stay in place as the ship rolled and started to sink around them, or don life vests and jump into the chilly ocean water miles from the South Korean coast.
At least some of those who jumped or made their way to the top of the ship were rescued. Helicopters crews plucked some from the deck.
Others were pulled from the water by crews aboard the multitude of fishing boats and military vessels that raced to the scene as the ship rolled over and capsized.
"I had to swim a bit to get to the boat to be rescued," Lim Hyung Min, one of more than 300 Seoul high school students who was aboard the ship for a four-day field trip, told CNN affiliate YTN. "The water was so cold and I wanted to live."
CNN affiliate YTN cited South Korean emergency response officials in reporting that six people were dead. The semiofficial Yonhap News Agency was still reporting four deaths early Thursday, mirroring what officials had told CNN on Wednesday.
At least 164 had been rescued, although some media reports put the number at as many as 179.
Still, nearly 300 people remained missing. A massive rescue operation resumed early Thursday after a delay of several hours, the South Korean Coast Guard told CNN. The reason for the delay was unclear.
Dozens of military divers, sailors, marines and police officers were assisting in the effort.
But cold water, swift currents and low visibility appeared to be complicating the rescue operation.
Water temperatures in the area are between 50 and 55 degrees Fahrenheit CNN meteorologist Brandon Miller said.
Divers from the South Korean navy searched three of the ship's compartments but found no survivors or bodies, Yonhap reported.
The U.S. Navy ship USS Bonhomme Richard, on routine patrol in the area, diverted to the scene and was standing by in case South Korean officials ask for help, said Lt. Arlo Abrahamson, spokesman for the U.S. Navy in South Korea.
"The Republic of Korea has done a great job in their rescue efforts thus far," he said.
Rescuers are "up against every sort of obstacle," said David Gallo, director of special projects at the Woods Hole Oceanographic Institution.
"It's just an absolutely, positively horrific situation," he said. "It's nightmarish."
A tilt and a bang
The ship ran into trouble just before 9 a.m. as it steamed toward to Jeju, a resort island considered the Hawaii of Korea.
Passenger Kim Sung-Mook told YTN that he was eating breakfast in the ship's main hall when he felt the ferry begin to tilt.
Someone made an announcement telling passengers to stay in place, that it would be dangerous to move, he said. Then, he said, he heard a loud bang.
Lim, the rescued student, told YTN he heard the bang before the ship began to list. The tremors knocked shipping containers off balance, he said.
"The students were falling over and crashing into things and bleeding," Lim said.
He obeyed orders to stay on the ship until rescuers arrived, threw him a life jacket and told him to jump.
The water, he said, was "unbearably cold."
What caused the ship to sink is unknown. It capsized within two hours of its first distress call, which came just before 9 a.m., Yonhap reported
Peter Boynton, a retired U.S. Coast Guard captain, said the speed with which the ship sank suggested it had sustained "major damage." He also said that if the ferry's car deck had been breached, it could have quickly swamped the ship.
The bodies of at least four people -- a female and three males-- were confirmed dead.
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'
The ferry, Sewol, was carrying 325 students; 15 teachers; 30 crew members; and 89 other passengers.
The students were from Seoul's Ansan Danwon High School. They 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 started 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."
Soon, the 6,800-ton ferry sank. Only its white and blue hull remained above water.
A desperate dash
With the clock ticking, rescue crews dashed desperately to get to passengers.
At least 178 people are involved in the rescue work, including 118 marines, 42 navy personnel and 18 police officers, Kang said.
The U.S. Navy ship USS Bonhomme Richard joined the operation and was headed to the scene. It sent helicopters to the area, the U.S. Navy said.
Dive teams have been going in and out of the submerged ship looking for bodies. But poor visibility has made their task difficult.
At 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 Paula Hancocks contributed from Seoul; CNN's Frances Cha, Madison Park and Judy Kwon also contributed to this report.
Posted by Greg Palmer