North Korean leader Kim Jong Un has invited South Korean President Moon Jae-In to visit his country for a summit meeting “at an early date,” Moon’s office announced Saturday. The invitation was passed along to Moon by Kim’s adviser and sister, Kim Yo Jong, who sat down with the South Korean leader in Seoul on Saturday for the highest-level meeting between the two sides in years. Kim reportedly also handed Moon a personal written letter from her brother, though no details were provided on the content of the message. Moon, who has enthusiastically welcomed Kim and been seen making friendly gestures to her at the Winter Olympics, said the two Koreas should “work together to create the environment” to make North Korea’s proposed summit happen. The invitation appears to be the first hint of a diplomatic breakthrough after months of nuclear threats and missile launches, though other world leaders, including Japan’s Shinzo Abe, have warned Moon not to buy into North Korea’s “smile diplomacy” during the games in Pyeongchang.