Ukraine rushed reinforcements to its northeastern Kharkiv region to hold off a Russian attempt to breach local defences.
It is a tactical switch that Kyiv officials have been expecting for weeks as the war stretches into its third year.
Intense nighttime shelling targeted Vovchansk, in the Kharkiv region and less than 5km (three miles) from the Russian border, regional governor Oleh Syniehubov said.
The barrage killed at least one civilian and wounded five others, prompting authorities to begin the evacuation of about 3,000 people.
Around dawn, Russian forces tried to pierce the Ukrainian defences near Vovchansk, Ukraine’s Defence Ministry said, adding that it had deployed reserve units to fend off the attack.
Analysts said the assault could mark the start of a Russian attempt to carve out a “buffer zone” that President Vladimir Putin vowed to create earlier this year to halt frequent Ukrainian attacks on Belgorod and other Russian border regions.
Ukrainian President Volodymyr Zelensky said that Ukraine’s military had anticipated the attack and had calibrated its response.
“Now there is a fierce battle in this direction,” Mr Zelensky was quoted as saying by Ukraine’s public broadcaster Suspilne.
Ukraine had previously said it was aware that Russia was assembling thousands of troops along the northeastern border, close to the Kharkiv and Sumy regions.
While the Kremlin’s forces have made their most recent ground thrust in eastern Ukraine, Ukrainian intelligence officials said they expected the Kremlin’s forces to attack in the northeast, too.
Though Russia likely cannot capture Kharkiv, Ukraine’s second-largest city, it could compel Ukraine to send more troops to the region, leaving other areas more vulnerable to attack. Also, forcing Ukrainian authorities to evacuate civilians is likely to create disruption and divert resources.
“The entire town is under massive shelling now, it is not safe to stay here,” Vovchansk administration head Tamaz Hambarishvili told Ukraine’s Hromadske Radio.
The Ukrainian Defence Ministry said fighting against Russian sabotage and reconnaissance groups continued into the afternoon.
Meanwhile, a Ukrainian long-range drone struck an oil refinery inside Russia on Friday, officials said, a day after what appeared to be the deepest strike by Kyiv’s forces on Russian soil hit a petrochemical facility.
Ukraine has repeatedly targeted refineries, hoping to disrupt the Kremlin’s war machine. Russia is one of the world’s biggest oil producers, providing key revenue and fuel.
A Ukrainian drone hit a refinery near the city of Kaluga, southwest of Moscow, setting four oil storage tanks ablaze, according to Vladislav Shapsha, the regional governor. He said there were no casualties.
The Russian Defence Ministry said that air defences downed seven Ukrainian drones early on Friday in the Moscow, Bryansk and Belgorod regions.
On Thursday, a senior official in Russia’s Bashkortostan region, about 1,300 kilometres (800 miles) from the Ukrainian border, said a drone strike in the city of Salavat caused a fire at a petrochemical facility.
The Russian Emergencies Ministry said that a pumping station building on refinery land was damaged, but there was no fire. Ukrainian military intelligence refused to comment.