North Korea has made “various changes” to military forces facing South Korea’s border islands, the commander of the ROK Marine Corps said Tuesday, emphasizing that Seoul has stepped up surveillance along the maritime border.

Marine Corps Commandant Gen. Ju Il-suk said at a parliamentary audit of the navy and marines that the changes involved the Korean People’s Army’s 4th Corps, which faces the islands. He declined to give details, citing operational security.

Ju’s remarks came after Kim Yo Jong, the sister of North Korean leader Kim Jong Un, said in a statement late Mondaythat the North’s military leadership was