North Korean leader Kim Jong Un has sent a message to Russian President Vladimir Putin hailing ties between the countries as "an unbreakable relationship of comrades-in-arms."

The Wednesday edition of North Korea's ruling Workers' Party newspaper, Rodong Sinmun, reported that Kim sent the congratulatory message on Russia Day, which marks the country's declaration of sovereignty on June 12, 1990.

Kim praised Putin for his "correct leadership," which he said had helped the Russian people to build a "powerful and prosperous Russia while frustrating all challenges and sanctions and pressure of the hostile forces."

Kim also referred to his summit with Putin in Russia last September.

He said the relationship between the countries is steadily developing into one of higher-level ties.

At the summit, Kim invited Putin to visit North Korea.

Multiple diplomatic sources have told NHK that arrangements are now being made for the Russian president to travel to the North as soon as early next week.

The visit would be Putin's first to the country in 24 years.