The Pentagon speechwriter commented on the issue as deepening military co-operation between Pyongyang and Moscow has sparked speculation that Russia could potentially ask the DPRK to send its troops to help Russian occupiers in Ukraine.

"If I were the leadership of North Korean military personnel, I would question my choice about sending my forces as cannon fodder in the illegal war against Ukraine. And we've seen various casualties... but again, something we'll be watching", Ryder said at a press briefing.

He also commented on popular talk that South Korea needs to consider the nuclear option in the midst of military cooperation between Russia and the DPRK.

"Our policy has not changed as it relates to the denuclearization of the Korean Peninsula. We will continue to work closely with our allies in South Korea and Japan and others in the region to ensure security and stability throughout the Indo-Pacific," he added, referring to South Korea by its official name, the Republic of Korea.

• The United States, South Korea and Japan condemned the deepening military cooperation between the DPRK, and Russia, and reaffirmed their intention to strengthen diplomatic and security cooperation.