In one of the biggest Ukrainian attacks on Russia of the two-year war, about 1000 Ukrainian troops rammed through the Russian border in the early hours of August 6 with tanks and armoured vehicles, covered in the air by swarms of drones and pounding artillery, according to Russian officials.
Ukrainian forces swept through the fields and forests of the border towards the north of the border town of Sudzha, the last operational trans-shipping point for Russian natural gas to Europe via Ukraine.
President Vladimir Putin cast the attack as a "major provocation".
The White House said the United States - Ukraine's biggest backer - had no prior knowledge of the attack and would seek more details from Kyiv.
Russia's most senior general, chief of general staff, Valery Gerasimov, told Putin on Wednesday that the Ukrainian offensive had been halted in the border area.
But multiple pro-Russian military bloggers said the battles continued into Thursday and that civilians were being evacuated.
Russian President Vladimir Putin called the incursion a "major provocation". (EPA PHOTO)
"Sudzha is basically lost to us. And this is an important logistics hub," said Yuri Podolyaka, a popular Ukrainian-born pro-Russian military blogger, adding Ukrainian forces were pushing north towards Lgov.
"In general, the situation is difficult and continues to deteriorate, despite the fact that the pace of the Ukrainian offensive has noticeably dropped."
The Ukrainian army has remained silent on the Kursk offensive.
Some Russian bloggers criticised the state of border defence in the Kursk region, saying that it had been far too easy for Ukrainian forces to slice through them.
The battles around Sudzha come at a crucial juncture in the conflict, the biggest land war in Europe since World War II.
Kyiv is concerned that US support could drop off if Republican Donald Trump wins the November presidential election.
Trump has said he would end the war, and both Russia and Ukraine are keen to gain the strongest possible bargaining position on the battlefield.
Ukraine wants to pin down Russian forces, which control 18 per cent of its territory, though the strategic significance of the border offensive was not immediately clear.
Former Russian president Dmitry Medvedev said the Ukrainian attack was an attempt to force Russia to divert resources from the front and to show the West that Ukraine could still fight.
As a result of the Kursk attack, Medvedev said, Russia should expand its war aims to include taking all of Ukraine.
"From this moment on, the SVO (Special Military Operation) should acquire an openly extraterritorial character," Medvedev said, adding Russian forces should go to Odesa, Kharkiv, Dnipro, Mykolayiv, Kyiv "and beyond".
"We will stop only when we consider it acceptable and profitable for ourselves."
Gas was still flowing through Sudzha via the Urengoy–Pomary–Uzhhorod pipeline which carried about 14.65 billion cubic metres of gas in 2023, about half of Russia's gas exports to Europe.
Russia's National Guard said it had beefed up security around the Kursk nuclear power station, which lies about 60km northeast of the town.
