China on Monday urged all sides in the Ukraine war to de-escalate as Kyiv's forces pierced deep into Russia's Kursk border region.

Kyiv has deployed thousands of troops to the surprise operation, a Ukrainian security official said, seizing the battlefield initiative after months of slow Russian advances across the east.

The assault seemed to catch the Kremlin off guard, with Moscow's army on Sunday appearing to concede that Ukraine had been able to penetrate its territory by up to 30 kilometres in places.

In response to the offensive, Beijing on Monday urged all parties to follow "three principles for de-escalating the situation".

Those are "no expansion of the battlefield, no escalation of fighting and no fuelling the flame by any party", the Foreign Ministry said in a statement.

"China will continue to maintain communication with the international community and play a constructive role in promoting a political solution to the crisis," it added.

Beijing released a paper last year calling for a political settlement to the conflict. (AFP)