SEOUL: North Korea was silent Monday on South Korea's renewed call for the release of a fishing boat and four crew members which strayed across the maritime border four days ago.

"The North Korean side simply replied that the investigation was underway," said unification ministry spokeswoman Lee Jong-Joo, after maritime authorities of the two countries communicated with each other early Monday.

Seoul says the 29-ton squid fishing boat drifted into the North's waters off the east coast Thursday due to a malfunctioning navigation system.

Pyongyang on Saturday said the boat had "illegally intruded" into its territorial waters, its first direct official response to the incident.

"A relevant institution is conducting concrete investigation into it at present," the communist state's official news agency said.

The two countries have remained technically at war since their 1950-53 conflict, but have sometimes returned each other's craft in the past.

Two South Korean trawlers strayed into the North's waters in April 2005 and December 2006 and were returned after five days and 18 days respectively.

But tensions have been mounting this year after nuclear and missile tests by the North and tougher UN Security Council sanctions in response.

The North has since March 30 detained a South Korean worker at the Seoul-funded Kaesong industrial estate, accusing him of insulting its system and urging a North Korean worker to defect.

It is also holding two US journalists captured on its border with China. They were jailed for 12 years in June for an illegal border crossing and an unspecified "grave crime."