South Korea says several North Korean soldiers briefly crossed the heavily-fortified military demarcation line between the two hostile countries in an incident over the weekend. After South Korean border patrols fired warning shots, the North Korean soldiers immediately retreated across the border again, with no “unusual activity” reported from the Sunday incident, the Joint Chiefs of Staff in Seoul said on Tuesday. According to South Korean news agency Yonhap, citing the general staff, some 20 North Korean soldiers had temporarily entered South Korea. The South Korean military said it assumes…