"We plan to mobilise all possible means to search for and rescue possible survivors given the sea currents in waters there," Won Tae-Jae, the South Korean defence ministry spokesman, said.
However, the South Korean weather office said water temperatures in the Yellow Sea were just 3.7 degrees Celsius and the local Yonhap news agency quoted coastguard officials as saying people could not survive for more than three hours in waters between eight and 10 degrees.
The 1,200-ton Cheonan had been on a routine patrol when an unexplained explosion in the ship's stern tore the vessel in two.
South Korean officials said that there was no immediate evidence that North Korea was to blame in the incident, which took place in the Yellow Sea, near the nations' disputed maritime border.
However, that line of investigation was still open according to Won said.
"The possibility of the ship having been attacked will be investigated along with other possibilities," he said.
South Korean YTN television, quoting an unidentified presidential official, said the ship was well south of the border and North Korean ships were unlikely to be in the area.
Choi Won-Il, captain of the Cheonan, told relatives of those missing: "The ship was torn apart and the stern sank immediately."
"While I was reviewing an operation plan in my cabin, there was the sound of an explosion and the ship keeled to the right. We lost power and telecommunications," the South's Yonhap news agency quoted him as saying.
"I was trapped in the cabin for five minutes before my colleagues broke the window in and let me out. When I got out, the stern had disappeared."
Lee Myung-Bak, the South Korean president, has ordered a thorough investigation into the sinking and the rescue effort after a fourth round of emergency security meetings on Sunday.
A government source quoted by Yonhap said officials were investigating various possible causes: an attack by a North Korean torpedo boat, a mine laid by North Korea or an explosion of munitions aboard the ship.
The joint chief of staff spokesman said there were no abnormal military movements on the North Korean side of the disputed maritime border, which was the scene of deadly naval clashes in 1999 and 2002.
Paul Chamberlin, a former US naval attache to South Korea, told Al Jazeera: "If it becomes clear this was an attack from North Korea, a major escalation that would lead to general war is very unlikely."
In November, the navies of the two sides exchanged fire in the area. The South's officials said a North Korean patrol boat had retreated in flames but it was not known if there were any casualties. No South Koreans were hurt.
The North refuses to accept the maritime border known as the Northern Limit Line, which was drawn up by UN forces after the 1950-53 Korean war. It says the line should run further to the south.
In January, the North fired 370 artillery shells into the sea near the border, raising tensions between the two sides.