South Korea's navy rescued three North Korean fishing boats and seven fishermen after the vessels drifted over a disputed border in the West Sea, a military official from the South said on Friday, according to Reuters. Four men were plucked from two sinking boats south of the maritime border near the South Korean island of Baengnyeong on Thursday. The crew were returned to North Korea at the inter-Korean truce village of Panmununjom later on Friday, South Korean media reported. A third vessel was found drifting in the same area also on Thursday. Its three crew were sent back after being supplied with fuel. Fishing boats from both sides frequently drift over the Northern Limit Line (NLL), which is difficult to pinpoint without sophisticated equipment. The North does not recognise the boundary, arguing it was unilaterally drawn up by the United States at the end of the 1950-53 Korean War and should be much further south. Earlier this week, it was the scene of a spat between the two Koreas' militaries. Each blamed the other for firing artillery shells toward the NLL. Several skirmishes have occurred on the border over the past decade, including two attacks last year in which 50 South Koreans were killed.