KYIV, UKRAINE: Thousands of Ukrainian troops are taking part in an incursion into the Kursk region in Russia which aims to "stretch" and "destabilise" Russia, a senior Ukrainian security official has told AFP.

"We are on the offensive. The aim is to stretch the positions of the enemy, to inflict maximum losses and to destabilise the situation in Russia as they are unable to protect their own border," the official said late Saturday, speaking on condition of anonymity.

The Russian army earlier said Ukraine had deployed around 1,000 soldiers in the incursion which began Tuesday, appearing to catch the Kremlin off guard, as Ukrainian forces penetrated Russian defensive lines.

Asked whether 1,000 Ukrainian troops were taking part in the assault, the official said: "It is a lot more [...] thousands".

After days of official silence from the Ukrainian authorities, President Volodymyr Zelensky acknowledged the operation for the first time in his nightly address on Saturday, saying that Kyiv was "pushing the war into the aggressor's territory."