The South Korean government is stepping up efforts to ensure the safety of South Korean workers who travel to the inter-Korean joint industrial complex in the North Korean border city of Gaeseong.

A Unification Ministry official says the ministry has put up warnings at the South’s immigration office at the inter-Korean border on the Gyeongui Highway that urge South Koreans to exercise added caution during their visits to Gaeseong.

The immigration office is also broadcasting warning messages every 30 minutes.

The measures come after North Korea threatened on Saturday to consider shutting down overland passage across the inter-Korean border, taking issue with South Korean human rights organizations’ sending of balloons with anti-North Korean leaflets into the North.