North Korea fired 10 artillery rounds into the sea off its west coast on Monday, South Korean media quoted government sources in Seoul as saying.

The YTN cable news channel and Newsis news agency reported North Korea fired the rounds at around 5:30 p.m. (0830 GMT) into its own waters near the border with South Korea.

"All we know is that North Korea fired artillery rounds into its sea off the west coast, but we have no more details as of now," Lieutenant-Commander Yu Jae-il at the Joint Chiefs of Command in Seoul said by telephone.

Tension between the neighboring countries have risen sharply lately after an international investigation has blamed the North for the sinking of a South Korean warship, killing 46 sailors in what officials called the worst military attack on the South since the 1950-53 Korean War.

The unease between the Koreas was also exacerbated by a recent joint four-day naval exercise with ally the United States involving 8, 000 troops.

North Korea, angered over the drill, vowed to respond with "a sacred war and a powerful nuclear deterrence."

"They will face a costly consequence if they stick to the criminal activities ravaging peace and security on the Korean peninsula," North Korea's main Rodong Sinmun newspaper said in commentary carried by the official Korean Central News Agency.

On Sunday, North Korean authorities seized a South Korean fishing boat 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.