SEOUL - North Korean authorities seized a South Korean fishing boat this week because the vessel illegally intruded into the North's territorial waters, Pyongyang's state media said on Saturday.

The report came two days after a South Korean military official said the North had towed the fishing boat to one of its ports.

The incident comes at a time of chilling ties between the two Koreas and while analysts say Pyongyang is in the midst of a sensitive process of resolving the leadership succession in Asia's only communist dynasty.

A patrol ship of the Navy of the Korean People's Army captured one ship of South Korea on July 30 when it illegally intruded deep into the DPRK territorial waters in the East Sea of Korea, the North's official KCNA news agency said.

A relevant institution is conducting a concrete investigation into it at present, it added.

The South has asked its neighbor to allow the vessel and its crew of four to return.

Fishing and other light vessels from both sides have strayed into the waters of the other in recent years where crews have been returned. But hundreds of other fishermen are thought to be held by the North after being abducted off the coast.

In the latest case, the boat appears to have strayed across the border on the east coast of the peninsula where it was intercepted by a North Korean patrol boat, the South Korean military official said on Thursday.

North Korea, already a pariah state, has become even more isolated by the international community in recent months after a series of missile launches and its second nuclear test which resulted in tighter U.N. sanctions.

(Reporting by Cheon Jong-woo; Editing by Dean Yates)