In one of the biggest Ukrainian attacks on Russia of the two-year-old war, around 1000 Ukrainian troops rammed through the Russian border in the early hours of Tuesday with tanks and armoured vehicles, covered in the air by swarms of drones and pounding artillery, according to Russian officials.
Heavy fighting was reported on Thursday near the town of Sudzha, where Russian natural gas flows into Ukraine, raising concerns about a possible sudden stop to transit flows to Europe.
The incursion has come as a shock to Russia, nearly two-and-a-half years since President Vladimir Putin sent his army into Ukraine in February 2022.
Ukraine soldiers fire a grenade towards a Russian position at Chasiv Yar, Donetsk region, Ukraine. (AP PHOTO)
Putin has cast the Ukrainian offensive as a "major provocation". Sergei Mironov, leader of a Kremlin-loyal political party, called it a "terrorist attack" and "the invasion of an internationally recognised foreign territory".
Kursk's regional acting governor, Alexei Smirnov, said thousands of residents had been evacuated.
The White House said the United States - Ukraine's biggest backer - had no prior knowledge of the attack. Washington has asked Ukraine for details on their military objectives, said a US official, speaking on condition of anonymity, on Thursday.
"They are operating within the area north of the border where we have told them they can use US-provided weapons to defend themselves against Russian attacks," the official noted.
Russia's defence ministry on Thursday said the army and the Federal Security Service (FSB) had halted the Ukrainian advance and were battling Ukrainian units in the Kursk region.
"Units of the Northern group of forces, together with the FSB of Russia, continue to destroy armed formations of the Armed Forces of Ukraine in the Sudzhensky and Korenevsky districts of the Kursk region, directly adjacent to the Russian-Ukrainian border," the ministry said.
The Ukrainian military has remained silent on the Kursk offensive, though President Volodymyr Zelenskiy praised the Ukrainian army on Thursday for its ability "to surprise" and achieve results. He did not explicitly reference Kursk.
Some Russian bloggers said Ukraine's forces were pushing towards the Kursk nuclear power station, which lies about 60km northeast of Sudzha.
