Seoul's government urged North Korea Monday to free a South Korean fishing boat and its crew as soon as possible, after the seizure heightened months of tensions between the two sides.

The South accuses the North of sinking one of its warships earlier this year and is staging a major naval exercise as a warning to its neighbour, despite protests and threats of retaliation from Pyongyang.

It was unclear whether the weekend seizure of the 41-ton squid fishing boat was a response to the drill, or just an attempt to curb alleged illegal fishing.

The South's unification ministry said there had been no word from Pyongyang since the Daeseung 55 was detained in or near an exclusive economic zone proclaimed by the North in the Sea of Japan (East Sea).

The South's coastguard has said the boat, which was being towed Sunday to the North's port of Songjin, was presumed to have been in the zone when seized.

"The government has urged the North to deal quickly with the case and release our crew members and their boat in accordance with international law and practice," said unification ministry spokesman Chun Hae-Sung.

It was unclear where the boat -- with four South Korean and three Chinese crew -- was located when it was seized, he said.

China's embassy in North Korea has expressed concern over the reported detention of the boat and crew, Xinhua news agency said.

If the report was confirmed, North Korea "should treat the Chinese crew members well with humanitarianism, guarantee their rights and interests, and inform the Chinese side," the Chinese agency quoted the diplomats as saying.

Cross-border tensions have been high since South Korea and the United States, citing findings of a multinational investigation, accused the North in May of sending a submarine to torpedo a corvette with the loss of 46 lives.

The North vehemently denies involvement in the March incident and says the current South Korean exercise is a prelude to an attack.

Military officers from the North and the US-led United Nations Command were Tuesday to hold their fourth round of talks on the warship sinking at the border truce village of Panmunjom.

The UN Command has been based in the South since the end of the 1950-1953 Korean War to enforce the armistice which ended the conflict.

At previous rounds the North demanded the right to send investigators to the South to inspect evidence dredged from the seabed, including what Seoul says is a part of a North Korean torpedo.

South Korea has rejected the demand, saying the UN Command should handle the case as a serious breach of the armistice.

The South's Joint Chiefs of Staff said the last day of the naval exercise in the Yellow Sea would feature an attempt to intercept a mock North Korean submarine.

Troops would also stage a live-fire drill at Baengnyeong island near the disputed sea border.

The overall exercise, which involves 4,500 troops, 29 ships and 50 fighter jets, was set to end late Monday evening. It is one of a series planned in coming months.

South Korean fishing boats have previously crossed the border with the North, sometimes due to navigation problems, but have mostly been sent back.

"There have been three such cases in recent months but the ships were sent back soon," said coastguard spokesman Ji Gun-Tae. "For now, we are just waiting for news from the North."

The crew of the Daeseung 55 last contacted the South Sunday afternoon via satellite phone and said their boat was being towed to Songjin.

The unification ministry said it had no plans at present to try to contact the North about the fishing boat. Communications are in any case restricted.

The North severed communications channels, except for two military hotlines, in May after the South announced reprisals for the alleged attack on the warship.