Ukraine claimed it was now controlling nearly 1,000 square kilometers of Russian territory as its forces pummeled two border regions on Tuesday.
Kyiv's ground forces tried to smash through defensive lines in a bid to carve out even more territory in its biggest attack on Russian territory since the war began.
Ukrainian soldiers crossed the Russian border, about 530 kilometers (330 miles) southwest of Moscow, a week ago, in a surprise attack that President Vladimir Putin said was aimed at improving Kyiv's negotiating position ahead of possible talks and slowing the advance of Russian forces along the rest of the front.
Russia's Defense Ministry said its air defense units destroyed 12 drones over the Kursk region, one over the Belgorod region and one over Voronezh. Russian officials in Kursk and Belgorod put out warnings for missile attacks.
Russia and Ukraine have given vastly different estimates of the area of Russian territory under Ukrainian control, with Kyiv claiming 1,000 square kilometers (386 square miles) while Russian officials said the area was less than half that.
Moscow's war bloggers reported intense battles across the Kursk front as Ukrainian forces tried to expand their control near Lgov, Bolshoy Soldatskoye and Korenevo – though they said Russia was bringing in both soldiers and heavy weaponry and repelled some of the Ukrainian attacks.
The fate of the town of Sudzha, through which Russia pumps gas from Western Siberia through Ukraine and on to Slovakia and other European Union countries, was unclear. Gazprom said Monday it was still pumping.
Some Russian war bloggers and Ukrainian Telegram channels said the town was in the control of Ukraine, although Reuters was unable to independently verify the battlefield accounts and Russia has yet to give details on the situation in Sudzha.
About 14.65 billion cubic meters (bcm) of gas was supplied via Sudzha in 2023 – about half of Russian natural gas exports to Europe, or about 5% of EU consumption.
The Ukrainian attack has underscored how effective small highly mobile units can be against the vastly numerically superior Russian army, though by dedicating forces to Kursk, Ukraine could weaken other parts of the front.
Russia on Monday reported that it was gaining in other parts of the front, though Moscow has had to move reserves to halt the Ukrainian advance.
President Volodymyr Zelenskyy told Ukrainians in his nightly address that the operation was a matter of Ukrainian security and the Kursk region had been used by Russia to launch many strikes against Ukraine.
"Russia must be forced to make peace if Putin wants to fight so badly," Zelenskyy said.
Russia invaded Ukraine in February 2022 and now controls 18% of Ukrainian territory. Until the surprise attack on Russia, Ukraine had been losing territory to Russian forces despite hundreds of billions of dollars in U.S. and European support aimed at stopping and even reversing the Russian advance.
In Kursk, 121,000 people had already left or have been evacuated and another 59,000 were in the process of being evacuated, local officials said.
In Russia's Belgorod region, which borders Kursk, 11,000 civilians were also evacuated, the region's governor said.
Ukraine's air force said Tuesday that Russia launched 38 attack drones and two ballistic missiles on Ukraine overnight. The whole country was briefly under air raid alerts early Tuesday, but these have since been called off.