Kim Jong Un told North Korean soldiers that the South was a "foreign" country, state media reported Friday, saying Pyongyang had jettisoned any idea of reunification.

Despite remaining officially at war, the two Koreas have long defined ties as a "special relationship", not state-to-state relations, with a view to eventual reunification.

But Kim in January defined Seoul as his country's "principal enemy", and on Friday described ties with the South as an "evil relationship" that had ended with the detonation of roads between the two. 

After months of laying fresh mines and ramping up security on the border, Pyongyang this week blew up roads and railways linking it to the South, and said its constitution now defined the South as a "hostile" state.

"Our army should keep in mind once again the stark fact that (South Korea) is a foreign country and an apparent hostile country," Kim told the 2nd Corps of the Korean People's Army, state media said.

Dynamiting roads and railways this week means "the end of the evil relationship with Seoul," Kim said, plus "the complete removal of the... unreasonable idea of reunification". 

The North's army will strike back if needed "against the hostile country, not the fellow countrymen," he added, the official Korean Central News Agency (KCNA) reported.

The North last week held a key meeting of its rubber-stamp parliament, where experts had widely expected the constitution to be revised.

On Thursday, Kim also examined "important documents" outlining the North's "military action plans for coping with different developments of the situation," KCNA said.

Pyongyang's official newspaper Rodong Sinmun published photographs of Kim issuing orders in front of a large, blurred map, while high-ranking officials diligently took notes.

The current armistice agreement, which ended active fighting in the 1950 to 1953 Korean War is akin to "a truce between the two systems that assert claims over the entire Korean Peninsula," said Hong Min, a senior analyst at the Korea Institute for National Unification.

But "this system may lose its relevance" as North Korea might change the way it thinks about its borders, he told AFP. 

"Such a shift would represent a transition from a temporary military demarcation line under ceasefire to a formal border system between nations," he added.

Copyright dispute?

South Korea's military released video footage on Tuesday of North Korean soldiers dynamiting the roads and railways, and Seoul later said Pyongyang appeared to have used the footage in state media.

Kim Yo Jong, the North Korean leader's powerful sister and a key regime spokesperson, said that the image in question was "a screenshot from one of the video clips released by NBC, Fox News, Reuters and other foreign media." 

All Seoul-based foreign media received the footage from the South Korean military.

Citing an investigation, Kim Yo Jong then accused South Korean media outlets, including the official Yonhap News Agency, of using images from Pyongyang's state media without authorisation.

Seoul's unification ministry said Friday that all South Korean "media companies legally use materials from the Korean Central News Agency by paying royalties through Japanese intermediaries", deputy spokesperson Kim In-ae told a briefing.

"We make it clear that it is North Korea that is unlawfully using our materials," Kim said. 

"As a member of the Berne Convention, North Korea should demonstrate a responsible attitude," she added, referring to the agreement designed to protect copyrighted works from infringement among member countries.

The nuclear-armed North has publicly bolstered military ties with Moscow in recent years.

Ukrainian President Volodymyr Zelensky said Thursday that he had intelligence reports saying North Korea was training 10,000 soldiers to support Russia in its fight against Kyiv.

"Without ruling out the possibility of this being true, we are sharing the necessary situation with related organisations and closely monitoring related trends," said deputy spokesperson Kim of Seoul.