Kursk, Moscow - Most of the 10,000 North Korean troops sent to Russia to aid it in its fight against Ukraine are already in action in the far western Kursk region, a US State Department spokesperson said on Tuesday.

Russia's success on the battlefield using North Korean troops "will in large part be dictated by how well the Russians can integrate them into their military," said the State Department's deputy spokesperson, Vedant Patel.

"Some of the challenges they would need to overcome are interoperability, the language barrier, command and control, and communications," he added.

Ukrainian President Volodymyr Zelensky has been warning that some of the 11,000 North Korean soldiers in the Russian border region of Kursk were involved in fighting with the Ukrainian army.

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According to Zelensky, Ukraine's advances in the region has tied down approximately 50,000 Russian soldiers who cannot be deployed to other Russian front positions on Ukrainian territory.

South Korea's spy agency on Wednesday also said North Korean soldiers were "engaging in combat" in Russia's Kursk region.

"The National Intelligence Service has assessed that North Korean troops dispatched to Russia have moved to the Kursk region over the past two weeks and have been deployed to the battlefield and are already engaging in combat operations," it said in a statement.

US Secretary of State Antony Blinken is expected to talk with EU and NATO representatives in Brussels on Wednesday about further support for Ukraine.