National Security Adviser Cho Tae-yong speaks to reporters at the presidential office in Seoul, March 30. (Photo: Yonhap)

Cho made the remark during an interview with Yonhap News TV, warning the North will be met with stronger sanctions if it goes ahead.

"North Korean State Affairs Commission Chairman Kim Jong-un announced it would come by the end of April, but he isn't keeping his promise," Cho said, citing "various" possible problems.

Cho said Seoul would respond by pressing the international community to strictly enforce sanctions on North Korea, while also imposing additional standalone sanctions together with like-minded nations.