SEOUL (AP) -- North Korea refused to give details Monday about four South Korean fishermen seized last week after their boat strayed across the border, only repeating that an investigation was under way, a South Korean official said.
The seizure on Thursday came amid badly frayed relations between the two Koreas and heightened tensions following Pyongyang's flurry of nuclear and missile tests in defiance of UN resolutions.
The communist regime also has been holding another South Korean citizen and two American journalists since March. Fears have grown that the fishermen could also be held long term if Pyongyang is tempted to use the case as a pressure card against Seoul.
On Monday, Seoul's Unification Ministry spokeswoman Lee Jong-joo said the South had queried the North about the case but was given the same answer as before.
"The North only said a corresponding agency is investigating" the case, she told reporters.
South Korea's government and families of the fishermen have urged Pyongyang to free the crew, saying their crossing into northern waters was accidental. The 29-ton vessel drifted across the eastern sea border after its satellite navigation system apparently malfunctioned, officials said.
On Saturday, the North's official Korean Central News Agency said that the boat "illegally intruded deep" into the North's territorial waters and that an investigation was under way.
The brief dispatch did not give any word on the fishermen's condition or say how long the probe would last.
The two Koreas technically remain at war because their three-year conflict ended in a truce in 1953, not a peace treaty. Their relations have been tense since a pro-US, conservative government took office in Seoul last year advocating a tougher policy on the North.
Pyongyang cut off nearly all ties in retaliation, and it has halted major joint projects except for an industrial complex located just across the border in the North. A South Korean worker at the factory park has been detained in the North since March for allegedly denouncing Pyongyang's political system.
The North has also been holding two US journalists after arresting them near the North's border with China on March.
Laura Ling and Euna Lee, reporters for former US Vice President Al Gore's Current TV media venture, were sentenced in June to 12 years of hard labor for entering the country illegally and engaging in "hostile acts."