Kim Yo Jong, the influential younger sister and key adviser to the leader, issued the warning with inter-Korean ties in a deep freeze despite three summits in 2018 between her brother and the South's President Moon Jae-in, who has consistently promoted engagement with Pyongyang.

North Korean defectors and other activists have long flown balloons across the border carrying leaflets that criticise Kim over human rights abuses and his nuclear ambitions.

"The South Korean authorities will be forced to pay a dear price if they let this situation go on while making all sort of excuses," Kim Yo Jong said in a statement carried by the official KCNA news agency.

Calling the defectors "human scum" and "rubbish-like mongrel dogs" who betrayed their homeland, she said it was "time to bring their owners to account" in a reference to the South Korean government.

Hours later, Seoul's unification ministry said the government was considering legislation to ban leaflet campaigns on the grounds they caused tension at the border.

"Any act that could pose a threat to the life and property of the residents in the border area should be stopped," said ministry spokesman Yoh Sang-key.

The leaflet campaigns have been a thorny issue between the two Koreas, but enacting such a law could spark an outcry over the possible infringement of the right to freedom of expression.

Nonetheless, an official at the South Korean presidential office said the leaflets did "more harm than good", adding the government would "respond firmly" to anything that damages national security, the Yonhap news agency reported.

In her statement, Kim Yo Jong threatened to scrap a military pact signed during Moon's visit to Pyongyang in 2018 aimed at easing tensions, and shut down a cross-border liaison office.

But most of the deals agreed at that meeting have not been acted on, with Pyongyang largely cutting off contact with Seoul following the collapse of a summit between Kim and US President Donald Trump in Hanoi last year that left nuclear talks at a standstill.

Operations at the liaison office have already been suspended because of the coronavirus pandemic, and North Korea has carried out dozens of weapons tests since the military agreement was signed.

Kim Yo Jong also threatened to pull out permanently from joint projects with South Korea, including the Kaesong Industrial Park and Mount Kumgang tours -- both of them money-spinners for Pyongyang that have been suspended for years due to sanctions over its weapons programmes.