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 (about 10 to 13 degrees Celsius), 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.”

Massive damage?

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.