South Korea’s Joint Chiefs of Staff said its troops fired warning shots at about 3 p.m. local time on Tuesday after several North Korean soldiers briefly crossed the military demarcation line inside the heavily fortified Demilitarised Zone. The North Korean troops, who were working on border-fortification projects, withdrew immediately and no casualties or damage were reported, Seoul added. North Korea’s state-run Korean Central News Agency quoted Lt. Gen. Ko Jong Chol, vice chief of the General Staff, as calling the firing of more than 10 rounds a “deliberate provocation” and warning that the North would take “corresponding countermeasures” if its work on sealing the frontier is impeded. He said the incident could push the border situation toward an “uncontrollable” phase. The episode comes as Pyongyang accelerates efforts to erect a permanent barrier along the inter-Korean frontier and amid annual US-South Korean military exercises that the North routinely denounces as rehearsals for war. It is the second such crossing reported this year, underscoring the challenge facing newly elected South Korean President Lee Jae Myung, who has pledged to reduce tensions and restore dialogue with the North.