North Korean authorities seized a South Korean fishing boat Sunday in waters off the divided peninsula's eastern coast, the South's coast guard said amid heightened tensions over the sinking of a southern navy ship.

Four South Korean and three Chinese fishermen were questioned Sunday for an alleged violation of the North's exclusive economic zone, South Korea's coast guard said in a statement. It said the fishing boat was being taken toward the North Korea's eastern port of Songjin.

The coast guard said it was not clear where exactly the fishing boat was operating when it was seized.

South Korea called on the North to quickly return the fishing boat and its crew. However, the prospect of their quick return is being complicated because of tension over the March sinking of a South Korean warship off the western coast blamed on North Korea.

South Korea also conducted naval drills off the western coast, including areas near the two countries' disputed sea border. The exercises, which end Monday, were aimed at strengthening South Korea's ability to counter any North Korean provocations.

North Korea - which has denied involvement in the sinking - warned last week it would counter the reckless naval firing projected by the group of traitors with strong physical retaliation and advised civilian ships to stay away from the maritime border.