North Korea yesterday said a South Korean fishing boat it seized had "illegally intruded" into its territorial waters in its first direct official response to the incident.

The South says the boat drifted into the North's waters off the east coast Thursday due to a malfunctioning navigation system, and called for an early return of the ship and its four crew members.

The North has yet to assure the South that the fishermen and the 29-tonne squid fishing boat will be returned.

"A patrol ship... captured one ship of South Korea on July 30 when it illegally intruded deep into the DPRK (North Korea) territorial waters in the East Sea of Korea," Pyongyang's official Korean Central News Agency said.

"A relevant institution is conducting concrete investigation into it at present."

The North's military on Friday faxed Seoul's government saying it was investigating the case, according to South Korean officials.

Yesterday's brief two-sentence dispatch marked Pyongyang's first direct official reaction to the incident.

Seoul's Unification Minister Hyun In-Taek expressed hope last Friday that Pyongyang would return the captured fishing boat "at an early date" together with its crew members, insisting the encroachment had been accidental.

Hyun described the North's relatively quick responses to the incident as "positive", raising hopes the crew would soon be released.

Ministry data showed two South Korean trawlers strayed into the North's waters in April 2005 and in December 2006, and they were returned five days and 18 days respectively after the seizures.

But tensions, which date back to the 1950-1953 Korean War, have been mounting this year after nuclear and missile tests by the communist state.

Pyongyang quit six-party talks aimed at ending its nuclear weapons programme after the UN Security Council censured it for a long-range rocket launch in April. Its second nuclear test followed a month later.

The Security Council has since imposed tougher sanctions.

The United States has urged the international community to continue to pressure North Korea to return to the six-party talks - made up of the two Koreas, the US, China, Japan and Russia.