Vladislav Shapsha, governor of the Kaluga region, which borders the Moscow region to its northeast, said a non-residential building in the region was on fire as result of Ukraine's drone attack. Reuters could not independently verify the reports. There was no immediate comment from Ukraine.

Experts warn that Russia may be trying to deplete Ukraine's air defences in preparation for an attack on its energy system over the winter. Ukraine has also increased its use of long-range drones in recent weeks,

The surge in fighting occurs as President-elect Donald Trump pledges to end the conflict.