Ukraine's military chief says his forces have seized more areas in their offensive in Russia's western region of Kursk.

Commander-in-Chief Oleksandr Syrskyi announced on Thursday that his forces had advanced 35 kilometers into Kursk since they launched the operation in the region. The cross-border incursion began on August 6.

Syrskyi said his forces had gained control of more than 1,150 square kilometers of Russian territory and 82 settlements in the offensive. He added that Ukraine has also set up a military commandant's office in Kursk.

But Syrskyi also said his forces were able to advance 0.5 to 1.5 kilometers on Thursday.

A US think tank, the Institute for the Study of War, quotes Russian military affairs bloggers as claiming that the Ukrainian operation in Kursk is generally slowing amid arrivals of unspecified Russian reinforcements.

The acting governor of Kursk, Alexei Smirnov, announced the expansion of evacuation areas on Thursday.

Russian defense minister Andrei Belousov said additional measures would be implemented to protect people and infrastructure from attacks by Ukrainian forces.

Russia's defense ministry released a video on Wednesday that it says shows a group of Ukrainian soldiers taken prisoner in Kursk.

Reuters news agency cites a source in the Security Service of Ukraine as saying Ukrainian special forces captured a group of more than 100 Russian soldiers in Kursk on Wednesday.