Thousands of Ukrainian troops on incursion to Russia: official

Thousands of Ukrainian troops are taking part in a major incursion into Russian territory, a top Ukrainian 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 security official said on condition of anonymity.

After days of official silence, President Volodymyr Zelenskyy acknowledged the offensive for the first time in his nightly address on Saturday, saying that Kiev was "pushing the war into the aggressor's territory".

Russia has launched its offensive on Ukraine in February 2022 and taking control of swathes of eastern and southern Ukraine and targeting Ukrainian cities with 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 Tuesday in the largest offensive by Kiev so far.

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 more than 76,000 civilians from the area, while Ukraine said last week it needs to evacuate 20,000 from the Sumy region across the border.

Morale boost for Ukraine

Russia's defence ministry on Sunday published footage it said was of its troops destroying Ukrainian military equipment that had advanced into its western Kursk region.

The offensive is now into its sixth day.

The attack has "greatly raised our morale, the morale of the Ukrainian army, state and society", the Ukrainian official said, speaking late Saturday 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".

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".

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

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".