On Friday, South Korean naval vessels rammed four North Korean boats, damaging three of them, in what Seoul's joint chiefs of staff called "a bump and push tactic". Correspondents say the confrontation almost ended in a firefight, after each side trained their weapons on the other. Each side has accused the other of crossing the Maritime Demarcation Line, drawn up by the United Nations Command (UNC) at the end of the 1950-53 war. The North said the South Korean authorities were engaged in "reckless provocations", despite repeated warnings, and warned that it would launch "self-defensive strikes" to protect its sovereignty. South Korea's Yonhap news agency, quoting naval sources, said on Saturday that, should North Koreanboats make a pre-emptive attack, the Navy would launch an immediate counter-attack to sink them. The area in question, midway between the North Korean mainland and five islands owned by the South, is only 100 km (60 miles) from Seoul. North Korean patrol boats accompany fishing vessels into the zone during the crab-fishing season, so the poverty-stricken communist state can earn some much-needed hard currency. Correspondents say the North's patrol boats usually withdraw when confronted by the South's navy, but on Friday, they refused to back down. Pyongyang declined a second invitation from UNC to hold talks with Seou at the UN truce village.