"We now assess that there are some 10,000 North Korean soldiers in total in Russia, and the most recent information indicates that as many as 8000 of those North Korean forces have been deployed to the Kursk region," Antony Blinken said in Washington.
"We've not yet seen these troops deploy into combat against Ukrainian forces, but we would expect that to happen in the coming days," Blinken added.
A combat deployment would make the troops "legitimate" targets in the war, he said.
It is the first time in 100 years that Russia has invited foreign troops into its country.
Where North Korean troops might deploy is unclear.
It also remains unclear whether North Korean soldiers could be deployed to fight on Ukrainian territory or in areas of Russia occupied by Ukraine, which includes parts of Kursk.
According to UN diplomats, it is also possible the North Koreans could be deployed behind the front line to carry out logistical tasks, for example.
South Korean intelligence had already said North Korean troops in Russia were wearing Russian uniforms and had been given false identities to conceal their true origin.
Russia has been waging a war against neighbouring Ukraine for more than two and a half years. (AP PHOTO)
Russian President Vladimir Putin does not deny the presence of North Korean soldiers and points out Ukraine uses personnel from NATO countries.
According to authorities, industrial plants in the Russian republic of Bashkortostan in the foothills of the Ural Mountains have been attacked by Ukrainian drones.
Window panes at an energy company were shattered and two other hostile drones crashed in the industrial area, the head of the republic, Radiy Khabirov, said on Telegram on Thursday. There were no casualties.
The capital of the republic, Ufa, is more than 1300 kilometres from the Ukrainian border.
Russia has been waging a war of conquest against neighbouring Ukraine for more than two and a half years, systematically targeting civilian areas in the hinterland.
Ukraine has started using its drones to attack targets in Russia further and further from the border.
At least two people were killed in a Russian glide bomb attack on a high-rise building in Kharkiv in eastern Ukraine, local authorities said on Thursday.
One of the dead is a 12-year-old boy, regional governor Oleh Synyehubov announced on Telegram. At least 34 people have been injured. Rescue workers searched the destroyed apartments for further possible victims.
President Volodymyr Zelenksiy called on foreign supporters to help his country urgently.
"Under these circumstances, every decision you postpone means at least dozens or even hundreds of such Russian bombs on Ukraine," he wrote on Telegram.
Other parts of Ukraine were also attacked from the air by Russia with missiles and combat drones during the night.
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