South Korea asked North Korea 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 North'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 North Korean maritime authorities, ministry spokeswoman Lee Jong-joo said. The ministry is responsible for handling relations with the North.

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

The seizure took place at about 6:30 a.m. Thursday when a North Korean patrol boat took the 29-ton vessel "800 Yeonan" into custody after it crossed into the North's waters due to apparent malfunctioning of its satellite navigation system, an official at South Korea'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 North Korean port.

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

North Korea has been holding a South Korean worker at the complex since March for allegedly denouncing its political system. South Korea has repeatedly demanded his release, but so far the North has not allowed Seoul officials any access to him.

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