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.