Kim Yo-jong, the sister of North Korean leader Kim Jong-un, has denounced South Korea for failing to stop defectors based in South Korea from sending anti-regime leaflets over the border last week.
In the statement on Sunday, she said that the South Korean government had not stopped what she called a "reckless act."
She called this a serious provocation, and said the North will look at taking corresponding action.
An activist group in South Korea said on Friday that it had released balloons carrying anti-North Korea leaflets and U.S. dollar bills several times last week.
South Korea has banned the sending of such leaflets into the North, and this was the first time the group has successfully defied the ban.