South Korea said on Monday that a border blast that seriously wounded two of its soldiers was caused by mines planted by North Korea, according to Reuters, in a text republished by WIFC.

Seoul says the mine exploded on September 21 and was believed to have been planted about a year ago, and demands an apology and the removal of the mines. On Friday, Kim Yo Jong, the sister of leader Kim Jong Un, said South Korea’s accusation was “groundless” and dismissed the demand for an apology.