North Korean leader Kim Jong Un said South Korea is a foreign and hostile nation, state media KCNA reported on Friday, with photos showing Kim conferring with high-ranking soldiers and poring over a map labelled "Seoul" at a command post.

Kim discussed the use of "offensive forces" against the South as retaliation for any move that infringes on its sovereignty, the report said. There was no elaboration on what forces might be used.

The report comes a day after KCNA said North Korea amended its constitution to designate the South as a "hostile state".

Kim has increasingly lashed out at South Korea this year, accusing Seoul of colluding with Washington to seek the collapse of his regime, and has pushed for a clear break with decades of policy engagement with the South, including the scrapping of unification as a goal.

The reclusive state blasted road and rail links with South Korea this week. Those actions underscored "not only the physical closure but also the end of the evil relationship with Seoul," KCNA quoted Kim as saying.

Seoul has said that if North Korea were to inflict harm upon the safety of its people, "that day will be the end of the North Korean regime."

Kim made the remarks while inspecting the headquarters of the 2nd Corps of the North Korean army on Thursday, KCNA said, a major military unit that includes several forward-deployed brigades with heavy weapons aimed at the South.

During the visit, he also said the changed nature of the South Korea-U.S. alliance, and their different, more developed military maneuvers highlight the importance of a stronger North Korean nuclear deterrent.

"Kim is trying to mentally fortify the frontline soldiers with his comments," said Yang Moo-jin, president of the University of North Korean Studies in Seoul.

"This 'two hostile countries' rhetoric is, in the end, Kim Jong Un's survival strategy... Don't interfere, live separately as a hostile country. It's a path (North Korea) has never gone before, and no one can be sure about its success."

Tensions between the Koreas have escalated since the North began flying balloons carrying trash across the border to the South in late May, with Seoul responding by restarting loudspeaker propaganda broadcasts, which anger Pyongyang.

North Korea has intensified its hostile rhetoric in recent days, accusing the South's military of flying drones over its capital on three days this month and threatening "a horrible disaster" if it detects another drone over its skies.

South Korea's government has declined to say if such drones were flown and if they were, whether they were flown by its military or civilians. It said to comment on the North's claim would be to get drawn into a ploy.

On Friday, the top military commanders of South Korea and the United States held an annual Military Committee Meeting and expressed concern over security challenges stemming from North Korea's threats.

"Both leaders underscored that the DPRK’s provocative acts, and the DPRK’s enhanced military cooperation with Russia destabilize peace and security on the peninsula and across the globe," the Joint Chiefs of Staff said in a statement, using North Korea's official name, the Democratic People's Republic of Korea.

On Sunday, South Korea will begin annual large-scale military exercises called Hoguk to improve operational performance.