The United States has expressed concern that Russia and North Korea appear to be actively advancing arms negotiations.

White House National Security Council spokesperson John Kirby told reporters on Wednesday that Russian President Vladimir Putin and North Korean leader Kim Jong Un have exchanged letters, pledging to increase bilateral cooperation.

Kirby said Russian Defense Minister Sergei Shoigu visited North Korea in July to try to convince Pyongyang to sell arms to Moscow. Shoigu headed a Russian delegation to the North to mark the 70th anniversary of the Korean War armistice.

Kirby added that there is also information indicating that another group of Russian officials traveled to Pyongyang after Shoigu's visit for follow-on discussions about potential arms deals between the two countries.

Kirby said that high-level discussions may continue in the coming months.

He warned that under these potential deals, Russia would receive significant quantities and multiple types of munitions from North Korea and the Russian military plans to use them in Ukraine.