North Korea threatened on Wednesday to "impose the death penalty" on the South's former president Park Geun-Hye over an alleged plot to assassinate its leader Kim Jong-Un.

Park had "pushed forward" a supposed plan by Seoul's intelligence services to eliminate the North's leadership, Pyongyang's security ministry and prosecutors said in a statement carried by its official Korean Central News Agency.

"We declare at home and abroad that we will impose death penalty on traitor Park Geun Hye," it said.

The former director of South Korea's National Intelligence Service (NIS) Lee Byung-Ho would meet a similar fate, it added.

They "can never make any appeal even though they meet miserable dog's death any time, at any place and by whatever methods from this moment".

The declaration comes after the killing of Kim's estranged half-brother Kim Jong-Nam by two women using the banned nerve agent VX at Kuala Lumpur international airport in February.

Both Malaysia and South Korea have blamed the North for the assassination, which retorts that the accusations are an attempt to smear it.

Last month Pyongyang's powerful ministry of state security said it had foiled a plot by the US and South Korean spy agencies to kill Kim using a biochemical weapon.

The lurid accusations came amid tensions over the North's nuclear and missile programmes and with Washington considering whether to re-designate Pyongyang as a state sponsor of terrorism.

The latest demand comes with Park's successor, new South Korean President Moon Jae-In - who backs engagement with the North - on his way to Washington for a summit meeting with Donald Trump.

The Pyongyang statement demanded that Seoul hand over "traitor Park" and the former intelligence chief "as they committed hideous state-sponsored terrorism against the supreme leadership" of North Korea.

Park is currently on trial in Seoul on charges of bribery and abuse of power related to the sprawling corruption scandal that saw her impeached.

The United Nations and rights groups accuse the North of widespread abuses, including an absence of fair trials.

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North Korean leader Kim Jong Un. (AAP)