SEOUL: North Korea has detained a South Korean fishing boat in the Sea of Japan (East Sea), the South`s coastguard said Sunday, in an incident which could further inflame high cross-border tensions.

The boat with four South Korean and three Chinese crewmen was seized while apparently operating near the North`s exclusive economic zone, the coastguard said in a statement.

It was being towed Sunday to the North`s northeastern port of Songjin.

"We urge North Korean authorities to handle this case in accordance with international norms and practices and return the ship and the crew at the earliest possible date," the coastguard said.

It was not immediately clear whether the Daeseung 55 was suspected of illegal fishing. The seizure came during a major South Korean naval exercise which has stirred anger and threats of retaliation from the North.

Cross-border relations have been stormy since South Korea and the United States, citing findings of a multinational investigation, accused it of torpedoing a South Korean warship in March.

The North denies involvement in the sinking and has threatened retaliation for what it calls a provocative South Korean military exercise being staged in the Yellow Sea in response to the sinking.

The exercise, which involves 4,500 troops, 29 ships and 50 fighter jets, is one of a series planned in coming months -- some of them with South Korea`s ally the United States -- in a show of force against the North.

YTN television said the fishing boat sent a message at 11:00 am Sunday (0200 GMT) saying it was heading for Songjin.

The television quoted a Seoul government official as saying the boat had been operating in or near a fishing area jointly shared between Russia and North Korea, 270 km (170 miles) east of the North`s Musudan region.

The vessel`s home port is Pohang in the southeast of South Korea.

The current naval drill follows a major joint South Korean-US naval and air exercise in the Sea of Japan late last month. It ended without incident despite the North`s threats of military retaliation.

As part of its own five-day drill due to end Monday, the South`s navy has staged intensive night-time exercises aimed at detecting North Korean submarines.

The navy came in for strong criticism for failing to detect the alleged submarine night attack on March 26 which split a corvette in two with the loss of 46 lives.

"The anti-submarine exercise... is a prelude to a war of aggression against the North," the newspaper of the North`s ruling communist party, Rodong Sinmun, said Saturday.

The North`s military has threatened "the most powerful" retaliation if the South triggers a conflict during the current drill. "Our warning is not empty talk," the paper said.

BDST: 1739 HRS, August 08, 2010