On Monday, a US Pentagon spokesman said the North Koreans had been killed without specifying the number, and a day later an unnamed US official said there were “several hundred” killed and wounded.
The BBC has not independently verified the claims.
The North Korean troops, none of whom will have any previous combat experience, are believed to have spent their first weeks in Russia in training and then in support roles.
The losses are believed to have occurred in the Kursk region of Russia, where Ukrainians are defending a small area of territory seized during a surprise invasion in August.
Last Saturday, Ukrainian President Volodymyr Zelensky said that Russia had begun using a “significant number” of North Koreans in its attacks on Kursk.
It is believed that they were not deployed in Ukraine itself, where Russian troops have been advancing in the eastern regions of the country in recent months.
