A North Korean submarine is missing and presumed sunk, according to reports in the US media.
The vessel was operating off the North Korean coast for several days when it disappeared, a paper with close links to the US military says.
The accident comes at a time of heightened tension in the region as South Korea and the US continue their largest-ever military exercise.
North Korean has issued another threat of war over the drill.
It said it was prepared to launch a pre-emptive strike in response to any sign that an invasion was being prepared.