A couple of thousand North Korean troops are moving toward Russia's Kursk region and a small number are already there, the Pentagon said on Tuesday, saying initial indications suggested Russia might field them in infantry roles against Ukrainian troops.

"We remain concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Kursk," Pentagon spokesperson Major General Patrick Ryder told reporters, adding he could not corroborate reports that North Korean troops were in Ukraine itself.

"We continue to monitor closely and are consulting with our Ukrainian partners, as well as other allies and partners."

Aditionally, US President Joe Biden said on Tuesday Ukraine should strike back if North Korean troops crossed into Ukraine, adding he was concerned by the presence of North Korean troops in the Kursk region in Russia.

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"If they cross into Ukraine, yes," Biden said when asked if the Ukrainians should strike back.