South Korea's Unification Ministry on Sunday urged North Korea to comply with inter-Korean agreements after Pyongyang threatened to take military action against Seoul.

The ministry said in a statement that the government is taking the current situations "gravely," stressing that Seoul and Pyongyang should adhere to all agreements.

The ministry's statement came a day after Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, threatened that Pyongyang will take a "next step" against the South and gave an instruction for the army to take necessary measures.

She also said that South Korea will soon witness the collapse of a useless inter-Korean liaison office.

Last week, North Korea called the South an "enemy" and vowed to cut off all inter-Korean communication lines, taking issue with the leaflet campaigns by North Korean defectors in South Korea.