Rescuers in ships and helicopters launched an intensive search Tuesday for 14 sailors feared drowned after their ship disappeared in freezing waters off South Korea.

The South Korean ship was carrying 2,000 tons of nitric acid, but it is unlikely to pose a threat to the ocean or marine life since the chemical dilutes easily, said Choi Eun-ju, a regional Coast Guard officer.

South Korea's Coast Guard and navy started searching for the ship shortly after it sent out a distress signal off Yeosu, about 280 miles south of Seoul, early Tuesday, Coast Guard spokesman Kang Byung-mun said.

Only one sailor -- identified as a 28-year-old Burmese -- has so far been rescued, while the remaining 14 crew members -- 12 South Koreans and two Burmese -- may have drowned, Kang said. Signals have not been detected from the ship for more than 15 hours.

The ship was sailing to Taiwan after leaving the southern port city of Gwangyang on Monday night, he said.

The survivor -- the only sailor wearing a life jacket -- told investigators he watched the ship sink before he was swept away by waves with his fellow crew members, Kang said.

"But we will continue our search and rescue efforts overnight," Kang said. "We're fighting against high waves and darkness." The Coast Guard mobilized 14 ships, two aircraft and three helicopters but only four ships would be used overnight, he said.

The news comes as South Korea battles to contain the nation's worst-ever oil spill, which has blackened beaches and jeopardized the ecosystem along the country's west coast. A wayward barge hit a supertanker on Dec. 7 causing the tanker to leak 78,920 barrels of oil.

The Coast Guard sent a boat to measure the acidity of the waters but it sailed back to shore due to high waves, said Song Hae-mi, another Coast Guard officer. A thin oil slick believed to be from the ship's fuel tanker was spotted on the waters.