South Korea says North Korea will investigate the crew of a South Korean fishing boat seized Thursday after it apparently strayed into North Korean territorial waters.

South Korea's Unification Ministry says it received word of the probe by a faxed message from Pyongyang.  According to the spokesman, "pertinent" authorities are looking into the matter.

The ministry says 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 to the eastern port of Jangjon early Thursday morning.  

Seoul is urging Pyongyang 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, says the fishing boat is relatively small, and made of reinforced plastic.  He says 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.