Russia's army on Sunday appeared to acknowledge Ukraine had pierced deep into its territory in the six-day offensive, saying it had hit Ukrainian troops and equipment in places around 30 kilometres (20 miles) from the border.
"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 security official said on condition of anonymity.
The shock offensive appeared to catch the Kremlin off guard.
The Ukrainian official also said Russia's claims that it had deployed 1,000 troops were a major underestimation of the forces Kyiv has committed to the operation.
"It is a lot more... Thousands".
After days of official silence, President Volodymyr Zelensky acknowledged the offensive for the first time in his nightly address on Saturday, saying that Kyiv was "pushing the war into the aggressor's territory".
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