KYIV, Ukraine — Ukraine rushed reinforcements to its northeastern Kharkiv region on Friday to hold off a Russian attempt to breach local defenses, authorities said, signaling a tactical switch in the war by Moscow that Ukrainian officials had been expecting for weeks.

Intense nighttime shelling targeted Vovchansk, in the Kharkiv region and less than 5 kilometers (3 miles) from the Russian border, regional Gov. Oleh Syniehubov said. The barrage killed at least one civilian and wounded five others, prompting authorities to begin evacuating about 3,000 people.

Then, around dawn, Russian infantry tried to penetrate Ukrainian defenses near Vovchansk, the Ukrainian Defense Ministry said, adding that it had deployed reserve units to fend off the attack.

Russian military bloggers 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.

Ukraine previously said it was aware that Russia was assembling thousands of troops along the northeastern border, close to the Kharkiv and Sumy regions. Although Russia's most recent ground offensive had been focused on parts of eastern Ukraine farther south, Ukrainian intelligence officials said they had expected an attack in the northeast, too.

President Volodymyr Zelenskyy said Ukraine's military had anticipated this latest attack and had calibrated its response.

“Now there is a fierce battle in this direction,” Zelenskyy was quoted as saying by Ukraine’s public broadcaster Suspilne.

Though Russia likely can’t capture Kharkiv, Ukraine’s second-largest city, it could compel Ukraine to send more troops to the region, leaving other areas of the country more vulnerable. Forcing Ukrainian authorities to evacuate civilians will also likely create disruption and divert resources.

The Russian military could also try to cut key supply routes in the area and try to blockade Kharkiv, which is home to roughly 1.1 million people and is only about 30 kilometers (about 20 miles) south of the border.

The Kremlin's forces are seeking to exploit Ukraine’s shortages of ammunition and manpower after the flow of Western military aid to Kyiv tapered off in recent months and before promised new support arrives.

The Ukrainian army is on the defensive along the roughly 1,000-kilometer (620-mile) front line and is scrambling to build fortified defensive lines ahead of what officials believe will be a bigger Russian offensive. Ukraine’s forces are outnumbered in infantry, armor and ammunition.

In the opening days of the war, Russia made a botched attempt to quickly storm Kharkiv but retreated from its outskirts a month later. Seven months after that, Ukraine’s army pushed the Kremlin’s forces out of Kharkiv in the fall of 2022. The bold counterattack helped persuade Western countries that Ukraine could defeat Russia on the battlefield and merited military support.

In Vovchansk, local officials said the Russian assault had damaged numerous buildings.

“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 Defense Ministry said fighting against Russian sabotage and reconnaissance groups had continued into the afternoon.

Meanwhile, Russian officials said a Ukrainian long-range drone struck an oil refinery inside Russia on Friday. The 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 Defense Ministry said air defenses downed seven Ukrainian drones early Friday in the Moscow, Bryansk and Belgorod regions.

The attack Friday came a day after one on a petrochemical facility in what appears to have been Ukraine's deepest strike into Russia. A senior official in Russia’s Bashkortostan region, about 1,300 kilometers (800 miles) from the Ukrainian border, said Thursday's drone strike in the city of Salavat caused a fire at the 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.

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.