Officials in Seoul say North Korea has seized a South Korean fishing boat that was operating in the East Sea (Sea of Japan) off the eastern coast of the Korean peninsula.

South Korea's coast guard said in a statement Sunday that Pyongyang is investigating the fishing vessel for possibly operating in North Korea's exclusive economic zone. The coast guard said it was not clear where the fishing boat was operating when it was seized.

The incident comes as South Korea holds a massive five-day anti-submarine exercise off its western coast that began last Thursday.

Thousands of soldiers and sailors are engaging in the drills in waters near where a South Korean naval vessel, the Cheonan, was sunk by an explosion in March.

The South blames the sinking on North Korea, which responded furiously to the exercise near the disputed maritime border. The North's official Korean Central News Agency quoted a government agency (the Committee for the Peaceful Reunification of the Fatherland) last week as threatening to use "offensive means beyond imagination" if the South ignites a war.

South Korea has promised an "immediate counter-attack" in case of any provocation from the North.

About 30 warships and submarines, 50 aircraft and 4,500 personnel are involved in the exercise, which runs through Monday.

The drills were scheduled in response to the Cheonan sinking, which killed 46 South Korean sailors. A five-nation investigation concluded that the ship was sunk by a North Korean torpedo fired from a submarine. Pyongyang denies any involvement and has demanded to inspect the evidence.

Some information for this report was provided by AP, AFP and Reuters.