North Korea said Saturday that a South Korean fishing boat it seized Thursday had entered its territorial waters illegally.
In its first official statement, Pyongyang's official Korean Central News Agency said a "relevant institution is conducting a concrete investigation" into the incident.
South Korea's Unification Ministry said Friday the fishing boat and its four man crew drifted into the North's territorial waters after it apparently experienced problems with its satellite navigation system. The boat was intercepted by a North Korean patrol boat and towed to the eastern port of Jangjon early Thursday.
Seoul is urging Pyongyang to release the boat and its crew. A spokesman for the Unification Ministry said Thursday there is a clear precedent for the boat's return, citing South Korea's return of two North Korean boats on humanitarian grounds after they crossed into Southern waters.
A spokesman for the South Korean military's Joint Chiefs of Staff, Park Sung-woo, said the fishing boat is relatively small, and made of reinforced plastic. He said that made it difficult for radar to detect the vessel before it crossed into North Korean waters.
Thursday's incident took place in waters west of the Korean peninsula - one of the tensest maritime regions in the world.
North Korea has never accepted a United Nations-mandated border there, and the two Koreas have fought two deadly naval battles in the area over the past 10 years.
North Korea is also holding two U.S. journalists and a South Korean worker at the Kaesong joint industrial plan across the border. Many in South Korea believe Pyongyang is trying to use the detainees as diplomatic leverage.
Some information for this report was provided by AFP, AP and Reuters.