South and North Korea traded artillery fire across their heavily militarised border on Thursday, in a rare exchange that left no casualties but pushed already elevated cross-border tensions to dangerously high levels. North Korea followed up with an ultimatum sent via military hot line that gave the South 48 hours to dismantle loudspeakers blasting propaganda messages across the border or face further military action. The South’s defence ministry dismissed the threats and said the broadcasts would continue. Thursday’s incident came amid heightened tensions following mine blasts that maimed two members of a South Korean border patrol earlier this month and the launch this week of a major South Korea-US military exercise that infuriated Pyongyang. In a detailed press briefing later in the day, the South’s defence ministry said the nuclear armed North initially fired a single artillery round over the border shortly before 4:00pm (0700GMT). Minutes later it fired several more in the rough direction of one of the South’s loudspeaker units, but the shells fell short on the South’s side of the demilitarised zone (DMZ)- a four kilometre wide buffer straddling the actual frontier line. The South Korea military retaliated by firing “dozens of rounds of 155mm shells” which the ministry said were also aimed to land in the North’s section of the DMZ. As a preventive measure, local South Korean residents in Yeoncheon county, some 60 kilometers north of Seoul were ordered to evacuate their homes for nearby shelters.