The warning came after NATO said that North Korean military units had been deployed to Russia's Kursk region to fight Ukrainian forces, the Reuters news agency reported.
The Pentagon estimated that 10,000 North Korean troops had been deployed to eastern Russia for training, according to Reuters.
"A portion of those soldiers have already moved closer to Ukraine, and we are increasingly concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Russia's Kursk Oblast near the border with Ukraine," said Pentagon spokeswoman Sabrina Singh, as cited by Reuters.
NATO Secretary-General Mark Rutte warned on Monday that "the deepening military cooperation between Russia and North Korea is a threat to both Indo-Pacific and Euro-Atlantic security."
He added: "It undermines peace on the Korean Peninsula and fuels the Russian war against Ukraine. Pyongyang has already supplied Russia with millions of rounds of ammunition and ballistic missiles that are fuelling a major conflict in the heart of Europe and undermining global peace and security."
Speaking after talks with a South Korean delegation in Brussels, Rutte told reporters that the deployment of North Korean troops "is also a sign of Putin's growing desperation."
He said: "Over 600,000 Russian soldiers have been killed or wounded in Putin's war, and he is unable to sustain his assault to Ukraine without foreign support."
'Lift restrictions on our long-range strikes against Russia': Ukrainian FM
Ukrainian Foreign Minister Andrii Sybiha said on X: "For weeks, Ukraine has been warning that Russia is preparing to deploy North Korean units. We haven’t seen a strong response. Now the NATO secretary-general has confirmed this."
Sybiha added: "The bottomline: listen to Ukraine. The solution: lift restrictions on our long-range strikes against Russia now."
Polish President Andrzej Duda last week expressed concern over North Korea's growing military cooperation with Russia.
Speaking after a meeting with South Korean leader Yoon Suk Yeol in Seoul on Thursday, Duda said: "We have firmly condemned North Korea for its nuclear and missile development, its provocations, and its illegal military cooperation with Russia."
He added that the deployment of North Korean forces to Russia poses a threat not only to the Korean Peninsula but also to global security.
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Monday is day 978 of Russia’s war on Ukraine.
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Source: IAR, PAP, Reuters, nato.int