North Korea has vowed to execute South Korea's former president and her spy director, accusing them of planning to assassinate its supreme leadership.

The official Korean Central News Agency said Wednesday that North Korea will impose a "death penalty" on ousted South Korean President Park Geun-hye and former spy chief Lee Byoung Ho, and they could receive a "miserable dog's death any time, at any place and by whatever methods from this moment."

South Korea's National Intelligence Agency (NIS) said the news report of a plot to kill Kim Jong Un "had no grounds" and it had no immediate comment about the North's demand for the handover of Park and her spy chief, Lee Byung-ho.

Lee is no longer head of the NIS.

Japan's Asahi Shimbun newspaper reported on Monday, citing sources familiar with Park's North Korea policy, that she had signed off on a plot to remove the North's leader in 2015 and the plan was orchestrated by the South's spy agency.

"We declare at home and abroad that we will impose the death penalty on traitor Park Geun-hye and ex-director of the puppet intelligence service ... criminals of hideous state-sponsored terrorism who hatched and pressed for the heinous plot to hurt the supreme leadership of the DPRK," KCNA said.

DPRK stands for the Democratic People's Republic of Korea, the North's official name.

"We declare that in case the U.S. and the South Korean puppet forces again attempt at hideous state-sponsored terrorism targeting the supreme leadership ... we will impose summary punishment without advance notice," KCNA said.

KCNA said the statement was issued jointly by the North's Ministry of State Security, the Ministry of People's Security and the Central Public Prosecutors Office.

North and South Korea are technically in a state of war under a truce that ended their 1950-53 Korean War and the North routinely warns of annihilating the South Korean government.