Ukrainian forces rammed through the Russian border last Tuesday and swept across some western parts of Russia's Kursk region, a surprise attack that laid bare the weakness of Russian border defences in the area.

Putin, in his most detailed public remarks on the incursion to date, said Ukraine was trying to improve its position ahead of possible talks.

He questioned what negotiations there could be with an enemy he accused of firing indiscriminately at Russian civilians and nuclear facilities.

"The main task, of course, is for the defence ministry to squeeze out, to knock out the enemy from our territories," Putin said, adding that Russian forces were accelerating their advance along the rest of the 1000km main front.

"The enemy will certainly receive a worthy response," he said.

He also said he expected further Ukrainian attempts to destabilise Russia's western border.

In his nightly address on Monday, Ukrainian President Volodymyr Zelenskiy said the war was now coming back to Russia after it had taken fighting to other countries.

He said that Ukraine's cross-border assault was a matter of security for Ukraine and that Ukrainian forces had captured areas from where Russia launched strikes.

His top commander, Oleksandr Syrskyi, said: "We continue to conduct an offensive operation in the Kursk region. Currently, we control about 1000 sq km of the territory of the Russian Federation."