South Korean ship hit in a possible torpedo attack
Rumours and denials are flying around over the incident, some claiming the attack might be a provocation by North Korea to start a new war on the peninsula.
The Yellow Sea maritime border, or Northern Limit Line, which is west of the Korean Peninsula, has seen other skirmishes in the past few years.
In 1999, 2002 and for the latest incident, in November 2009, ships from the navies of North and South Korea fired at each following real or imaginary breaches of the border.
In January, Pyongyang carried out some explosions in a military exercise around Baengnyeong and Daecheong Islands. Before the action, it banned all navigation near the islands.
The maritime border between the two Koreas was laid down at the end of the Korean War (1950-1953) but was never accepted by the North, partly because a peace treaty did not follow the armistice.
Technically, the two Koreas are thus still at war.