Thousands of Ukrainian troops are taking part in an incursion aiming to destabilise Russia by showing up its weaknesses, a top Ukrainian official has said as the assault entered its sixth day, Ednews informs via The Guardian.

“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 Russian army had said about 1,000 Ukrainian troops were deployed in the cross-border incursion which began on Tuesday and appeared to catch the Kremlin off guard, allowing Ukrainian forces to penetrate Russian defensive lines.

Asked whether the 1,000 figure was correct, the official said: “It is a lot more … Thousands.”

After days of official silence, President Volodymyr Zelenskiy acknowledged the offensive for the first time on Saturday, saying that Kyiv was “pushing the war into the aggressor’s territory”.

Russia invaded Ukraine in February 2022 and has waged an unrelenting offensive, occupying swathes of eastern and southern Ukraine and subjecting Ukrainian cities to daily missile and drone attacks.

After re-capturing large areas in 2022, Ukrainian forces have largely been on the backfoot and are increasingly struggling with manpower and arms supplies. But Ukrainian units stormed across the border on Tuesday in what has been, so far, the largest and most successful such offensive by Kyiv in the conflict.

Its troops have advanced several kilometres, forcing Russia’s army to rush in reserves and extra equipment – though neither side has given precise details on the forces committed. Russia has evacuated tens of thousands of civilians from the area and Ukraine has also evacuated thousands of people from the Sumy region across the border.

The operation has “greatly raised our morale, the morale of the Ukrainian army, state and society,” the Ukrainian official said, speaking after weeks of Russian advances in eastern Ukraine.

“This operation has shown that we can go on the offensive, move forward. It seems that the Russians have problems with coordination, preparedness for action,” he said.