Ukraine is staging its biggest cross-border attack into Russia since the start of the conflict, prompting Moscow to send additional units to defend its territory.

Ukrainian troops began its push into the region of Kursk in western Russia on Tuesday, and are believed to have seized a number of villages.

Russia's defense ministry said on Friday that additional tank units are being sent to Kursk, along with rocket launchers and artillery guns.

The US-based Institute for the Study of War said on Friday that the Russian military command may be transferring "more experienced and better-provisioned frontline units from eastern or southern Ukraine" to the Kursk region. It adds that "it would likely take additional time for such units to arrive."

Russia's defense ministry said on Saturday that its air defense had destroyed 26 Ukrainian drones over Kursk.

Russia also said what are believed to be fragments of downed missiles were found on Thursday inside the premises of the Kursk nuclear power plant.

Moscow accused Ukrainian forces of threatening the safety of the nuclear plant, and said it has reported the situation to the International Atomic Energy Agency.