North Korean soldiers have again crossed the heavily-fortified military demarcation line with South Korea in the third such incident between the two hostile countries in less than two weeks, Seoul said on Friday. After South Korean border patrols fired warning shots, the North Korean soldiers immediately retreated across the border again in the incident on Thursday, said the Joint Chiefs of Staff in Seoul. According to South Korean news agency Yonhap, citing the general staff, the soldiers were observed carrying out construction work within the demilitarized zone (DMZ). North Korea has deploye…