Western and Ukrainian officials have called the appearance of North Korean forces on the battlefield a major escalation.

President Volodymyr Zelenskiy called on Ukraine's allies to stop "watching" and take action to tackle the presence of North Korean troops in Russia before they start confronting his country in combat.

"We believe that the DPRK [Democratic People's Republic of Korea] has sent around 10,000 soldiers in total to train in eastern Russia," Deputy Pentagon press secretary Sabrina Singh said.