A South Korean military ship sank, Friday, near its disputed boarder with North Korea.
Officials say the ship was on patrol when an explosion ripped a hole in the stern.
South Korean news agencies reported the military is investigating whether the ship was hit with a torpedo from North Korea.
The 1,200-ton "Cheonan" had a crew of 104. So far, 58 sailors onboard have been rescued.
Meanwhile, North Korea has issued new threats, as it does on a regular basis, to strike the United States and South Korea with nuclear weapons.
The communist country is believed to have enough weapons grade plutonium for at least half a dozen atomic bombs.
The U.S. is leading efforts to persuade the North to give up its nuclear arms.
A top U.S. commander in the region says the possibility of turmoil in the area is of real concern.