Russian forces have battled Ukrainian troops for a third day after they smashed through the Russian border in the Kursk region, an audacious attack on the world's biggest nuclear power that has forced Moscow to call in reserves.
In one of the biggest Ukrainian attacks on Russia of the two-year war, about 1,000 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.
Russia's defence ministry said on Thursday 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.
Ukraine had lost 82 armoured vehicles including eight tanks in the attack, it said.
The Ukrainian army has remained silent on the Kursk offensive.
Some Russian bloggers criticised the state of border defences in the Kursk region, saying it had been far too easy for Ukrainian forces to slice through them.
"The enemy passed through our line of defence quite easily," said Yuri Podolyaka, a popular Ukrainian-born, pro-Russian military blogger, adding that no complete defensive work had been prepared in the Kursk region despite the ongoing conflict.
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 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.
Several thousand people were being evacuated from the Kursk region, according to Russian media.
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.
Russia's National Guard said it had beefed up security around the Kursk nuclear power station, which lies about 60km northeast of the town.
