SEOUL: South Korea Wednesday sent North Korea a message urging it to free the seven crewmen on a South Korean fishing boat seized last weekend, Seoul`s unification ministry said.

The South, in a faxed message through a military hotline, called on the North to release the boat and its crew "based on international laws and customs and humanitarian spirit", ministry spokeswoman Lee Jong-Joo told a briefing.

The seizure of the 41-ton squid fishing boat and its crew -- four South Koreans and three Chinese -- heightened months-old tensions between the two sides.

The message was sent in the name of South Korea`s Red Cross to its counterpart in the North, a form of communication used in the past when fishing boats have crossed the inter-Korean sea borders.

The South`s coastguard has said the boat was presumed to have been inside an exclusive economic zone proclaimed by the North in the Sea of Japan (East Sea) when it was detained.

There has been no word from Pyongyang on the incident. The seizure was made during a major South Korean naval exercise in the Yellow Sea, for which the North had threatened retaliation.

Cross-border tensions have been high since South Korea and the United States accused the North in May of torpedoing one of Seoul`s warships with the loss of 46 lives.

The North vehemently denies involvement in the warship incident. It says naval exercises staged in response to the sinking were a rehearsal for aggression.

Late Monday, just after the South`s latest naval drill ended, the North fired some 130 artillery rounds into the Yellow Sea near the disputed border.

Around 10 of the shells landed south of the borderline, prompting the South`s military Tuesday to vow a resolute response to future provocations.

China, North Korea`s sole powerful ally, has expressed concern over the detention of its nationals aboard the fishing boat.

BDST:8:37 HRS, August 11, 2010