SEOUL: The Republic of Korea (ROK) asked the Democratic People's Republic of Korea (DPRK) to quickly release a fishing boat and its four crew members, hours after the vessel was seized after accidentally crossing the countries' eastern sea border, an official said Thursday.

The incident comes amid tense relations between the countries over the DPRK's nuclear and missile programs and amid a steady deterioration in ties since early last year.

Seoul's Unification Ministry made the formal written request to the DPRK's maritime authorities, ministry spokeswoman Lee Jong-joo said. The ministry is responsible for handling relations with the DPRK.

The DPRK confirmed it received the request and told the ROK by telephone that it would look into the matter and inform the ROK later, Lee said. The DPRK's state news agency, monitored in Seoul, did not mention the incident.

The seizure took place at about 6:30 am Thursday (2130 GMT Wednesday) when a DPRK patrol boat took the 29-ton vessel "800 Yeonan" into custody after it crossed into the DPRK's waters due to apparent malfunctioning of its satellite navigation system, an official at the ROK's Joint Chiefs of Staff said.

The official, who spoke on customary condition of anonymity, said the seized boat's location was unknown and it was unclear if it had been towed into a DPRK port.

Relations between the DPRK and the ROK worsened last year after a pro-US, conservative government took office in Seoul, advocating a tougher policy on the DPRK. In retaliation, Pyongyang cut off ties and halted all major joint projects except a joint industrial complex located just across the border in the DPRK.

The DPRK has been holding a ROK worker at the complex since March for allegedly denouncing its political system. The ROK has repeatedly demanded his release.

Two ROK fishing boats accidentally crossed into the DPRK in 2005 and 2006, respectively, before the DPRK later released the ships and their crews on humanitarian ground, said Lee, the Unification Ministry official.