North Korea threatened Thursday to scrap a military agreement with the South and close down a cross-border liaison office unless Seoul stops activists from flying anti-Pyongyang leaflets over the border.

The statement issued by the powerful younger sister of North Korean leader Kim Jong Un comes amid a deep freeze in inter-Korean ties, despite three summits between Kim and the South's President Moon Jae-in in 2018.

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.

She threatened to scrap a military pact signed during Moon's visit to Pyongyang in 2018 aimed at easing border 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 the North 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 the South including the Kaesong Industrial Park and Mount Kumgang tours -- both of them money-spinners for the North that have been suspended for years due to sanctions over its weapons programmes.