U.S. officials said Tuesday they are «concerned» about reports of North Korean soldiers fighting in Ukraine on behalf of Russia.

This week, Ukrainian President Volodymyr Zelenskyy accused North Korea of transferring soldiers to Russia’s military, saying his intelligence agencies briefed him on «the actual involvement of North Korea in the war» in Ukraine.

The briefing came a week after Seoul’s defense minister, Kim Yong-hyun, told South Korean politicians it was «highly likely» six North Korean officers were killed in a recent Ukrainian missile strike on Donetsk.

Despite the reports, Russia has dismissed the allegations as «fake news.»

NORTH KOREAN TROOPS NOW FIGHTING FOR RUSSIA IN UKRAINE, SEOUL SAYS

Still, officials in the U.S. are taking notice of the reports.

«We are concerned by reports of DPRK [Democratic People’s Republic of Korea] soldiers fighting on behalf of Russia, which if true would mark a significant increase in the DPRK-Russia defense relationship,» said Lt. Col. Charlie Dietz, a spokesperson for the Department of Defense. «Such a move would also indicate a new level of desperation for Russia as it continues to suffer significant casualties on the battlefield in its brutal war against Ukraine.»

Also weighing in Tuesday was State Department spokesperson Matthew Miller, who echoed Dietz.

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A Ukrainian tank (Reuters/Lisi Niesner)

«We are concerned by the reports of DPRK soldiers fighting on behalf of Russia,» Miller said. «If that’s true, it would mark a significant increase in the relationship between those two countries, the relationship that you have seen develop over the past several months. It would also indicate a new level of desperation by Russia as it continues to suffer significant casualties.»

Washington says North Korea has supplied Russia with ammunition and ballistic missiles, though Moscow and Pyongyang have denied the arms transfers. But the two countries have not denied but instead have vowed to boost military ties, which could include joint drills.

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The ruins of apartment buildings are shown after a Russian military strike in Vuhledar, Ukraine. (Reuters/Yevhen Titov)

Russia and North Korea have deepened relations since Moscow’s invasion of Ukraine in February 2022. The two nuclear-armed states signed a defense pact in June when Russian President Vladimir Putin visited North Korea.

Both Russia and Ukraine have resorted to foreign fighters as their numbers of draft-age men dwindle.

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North Korea has recently ramped up production and testing of artillery and cruise missiles.

In March, Russia used its veto power on the United Nations Security Council to end monitoring for violations of a ban on North Korea testing ballistic technology.

Fox News’ Liz Friden and Morgan Phillips as well as Reuters and the Associated Press contributed to this report.