In June last year, North Korean troops violated the border three times, prompting South Korea to fire warning shots. The incidents occurred when the Koreas were embroiled in Cold War-style campaigns like balloon launches and propaganda broadcasts, but they didn’t develop into a major source of tensions.

South Korea’s military fired warning shots at a group of North Korean soldiers who crossed the border on Tuesday, according to Seoul government officials. It has been almost a year since the last report of troops from North Korea violating the heavily fortified demarcation line between the two countries.