A senior Ukrainian military source says Ukraine has lost more than 40 percent of the territory its troops seized in their incursion into Russia's western region of Kursk.

The Ukrainian General Staff source disclosed the information that was reported by Reuters news agency on Saturday.

The source told Reuters that Ukraine had taken about 1,400 square kilometers in its cross-border offensive in the Kursk region in August.

But the source added that Ukraine now controls around 800 square kilometers of the territory due to increased Russian counterattacks.

The source said that Ukraine would try to hold the land still under its control for as long as "militarily appropriate."

The source also confirmed to Reuters that about 11,000 North Korean troops had arrived in the Kursk region in support of Russia, but most of them were still finalizing their training.

Regarding the situation in Ukraine, the source revealed that Russian forces have been advancing 200 to 300 meters a day around the town of Kurakhove in the eastern region of Donetsk. Reuters says the town is a "stepping stone" toward the strategically important city of Pokrovsk.