South Korea said Sunday it believes that a North Korean fishing boat intentionally crossed the tense western maritime border into South Korean waters late last month. South Korea's Navy captured the fishing boat on March 27 after it ignored South Korea's repeated warnings to retreat back to the North's waters. South Korea thought at the time that the boat strayed into South Korean waters because of engine problems, a belief supported by the wishes of all three North Koreans on board to return to their homeland. South Korea repatriated the North Koreans and their boat in the wee hours of March 28. But South Korea has since changed its assessment of the incident, noting that there were no traces of any fish having been caught on the boat and its fishing net was also not adequate for fishing. In an unusual move, about 10 North Korean patrol vessels were also on standby just north of the sea border at the time the North Korean fishing boat crossed the border into South Korean waters, a South Korean government official said. The General Staff of North Korea's military claimed on March 28 that South Korean sailors beat its fishermen with iron sticks and later coerced them to settle in South Korea at gunpoint. The North's General Staff said its military will force South Korea's Navy "to pay a dear price" for what it claims were atrocities against the North Korean fishermen by the South Korean Navy. South Korea's Navy has expressed deep regret over the North's false accusation. The three North Korean fishermen said in a news conference in Pyongyang on March 29 that they were picking up clams before their boat stopped running due to engine problems amid thick fog. Last week, the rival Koreas exchanged fire near the sea border, the scene of several bloody clashes between them since 1999. The North does not recognize the sea border because it was unilaterally drawn by the U.S.-led U.N. forces at the end of the 1950-53 Korean War that ended in a cease-fire, not a peace treaty.