The surprise attack seeks to reverse Ukraine’s decreasing hold of the region. Since Kyiv’s forces gained partial control of Kursk following a daring cross-border assault last August, Russian troops, aided by North Korean soldiers, have halved the territory held by Ukraine, albeit while suffering thousands of casualties.

The Institute for the Study of War (ISW), a US-based think tank that maps the frontline, said Ukraine had made advances into the southern outskirts of Berdin. It is to the northeast of the city of Sudzha, the most significant area held by Ukraine since the start of the first assault five months ago.

Russia said Sunday that Ukraine had launched a "counterattack" in the western border region of Kursk, where Kyiv's forces began a shock ground offensive last August.- Airports shut - Both Russia and Ukraine have exchanged regular attacks since the year began.