North Korean leader Kim Jong Un's very influential sister has threatened its neighbor, South Korea, with military action as she lashed out at Seoul over collapsing bilateral ties and its incapacity to curb activists from dropping border-crossing anti-Pyongyang fliers.

Describing the South as its "enemy," Kim Yo Jong repeated an earlier threat by saying the city will soon see the fall of an inter-Korean "useless" liaison office in Kaesong's border town.

Kim Yo Jong, who serves unofficially as one of the closest advisers of Kim Jong Un, issued the warning in a statement released on Saturday by state news agency KCNA.

"By exercising my power approved by the Supreme Leader, our Party and the State, I gave instructions to the ... department responsible for the affairs with (the) enemy to carry out decisively the next action," said Kim, the Telegraph reported, citing a Reuters News Agency story. 

Yo Jong's statement, which did not disclose what the next military action might be, was released days after South Korea took legal action against defectors who sent rice and anti-North leaflets, usually by balloon over the heavily-guarded border or by sea using bottles.

The move by the North is the latest escalation of frictions over defectors from the country who have been sending back propaganda and other materials from their new homes in the South.

Pyongyang stated that it had been irked by the defectors and to show its disappointment, the country had in the past week cut inter-Korean communications and threatened to abandon important inter-Korean peace accord signed by their leaders in 2018.

Kim's blunt rhetoric confirms her elevated status in the country's leadership. Already known as North Korea's most powerful woman and the closest confidant to her brother, government media recently reported that she is now in charge of relations with South Korea.

Kaesong's liaison office, which has been shut down since January due to worries about COVID-19, was established as a result of one of the important deals reached at three summits between Kim Jong Un and South Korean President Moon Jae-in in 2018.

According to Duyeon Kim, a senior advisor at Belgium-headquartered International Crisis Group, North Korea feels misled by the South's projection that the United States would ease some restrictions in exchange for Pyongyang shutting down its nuclear reactor, and is disappointed that leaflets and US-South Korea military exercises continue.

Meanwhile, some experts said Pyongyang, which has mobilized people for large demonstrations that condemn defectors, is intentionally censuring Seoul to encourage its public and divert attention from a struggling economy, which has deteriorated during the coronavirus pandemic.