Ukraine has deployed thousands of troops in a major incursion into Russia’s western Kursk region, in a bid to “stretch” and “destabilise” Russia, a senior Ukrainian security official told AFP.

Ukrainian units stormed into Russian territory on Tuesday in a surprise attack that has developed into the most significant cross-border assault by Kyiv since the conflict began in February 2022.

“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 appeared to catch the Kremlin off guard with Ukrainian forces successfully penetrating Russian defensive lines.

Asked whether 1,000 Ukrainian troops were taking part in the assault, the official said: “It is a lot more… thousands”.

The offensive was also having an important psychological effect at home, he said.

The attack 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,” the official said.

“It seems that the Russians have problems with coordination, preparedness for action,” he said.

But he said there had been little effect so far on fighting in the east.

“The situation is basically unchanged. Their pressure in the east continues, they are not pulling back troops from the area,” he said, adding only that “the intensity of Russian attacks has gone down a little bit”.

No annexation plans

The official said Ukrainian troops would respect international humanitarian law while on Russian territory and had no plans to annex areas they currently hold.

“There is no idea of annexation… We are operating in strict accordance with international law,” he said, contrasting this with alleged violations by Russian troops in occupied territory.

Asked whether capturing the Kursk nuclear power plant near the border was an aim, he said: “We will see how the Kursk operation will develop.”

Russia has sounded the alarm over feared strikes on the plant, 60 kilometres (37 miles) from the Russia-Ukraine border.

The International Atomic Energy Agency has urged both sides “to exercise maximum restraint in order to avoid a nuclear accident with the potential for serious radiological consequences”.

“We absolutely will not cause problems for nuclear security. This we can guarantee,” he said.

The Ukrainian official also rejected reports that Kyiv’s Western partners had not been informed about the offensive.

The White House said Wednesday it was contacting Ukraine to learn more about the “objectives” of the incursion.

Asked whether Western partners had been kept in the dark about Ukraine’s offensive, the official said this was “incorrect”.

“Judging by how actively Western arms are being used, our Western partners played a part indirectly in the planning,” he said.

Russia’s top general has pledged to put down the offensive and drive Ukrainian troops back to the border.

The official said he expected Russia would “in the end” manage to stop Ukrainian forces.

He also expected Moscow would retaliate with a large-scale missile attack, including “on decision-making centres” in Ukraine.