A Japanese expert on North Korea says the country has blown up inter-Korean roads to emphasize that South Korea is a hostile nation, based on a change of its policy of seeking a peaceful unification.

The South Korean military said North Korea blew up sections of inter-Korean roads on its side of the border at around noon on Tuesday.

Keio University Professor Isozaki Atsuhito says North Korea's leadership needed a strongly symbolic event to change its people's mindset after abandoning its decades-long policy of national reunification.

Isozaki explains the message was that they must be aware the South is a foreign country, and that it has nothing to do with the North.

He says the North's leadership must have realized that it is unrealistic to pursue reunification efforts, and pivoted to defend its current regime.

The professor also says Russia's backing helps North Korea maintain its confrontational stance against the South. He points out that the North has become more aggressive since it signed a de-facto alliance treaty with Russia.