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.

Kharkiv’s regional governor, Oleh Syniehubov, said intense overnight shelling targeted Vovchansk, a city with a prewar population of about 20,000 that is less than 3 miles from the Russia border. The barrage, which used powerful guided aerial bombs, artillery, rockets, tanks and mortars, 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 border regions.

Ukrainian troops were still holding firm against the assault at midafternoon, Syniehubov said.

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. The Kremlin’s forces stepped up their bombardment of Kharkiv, Ukraine’s second-largest city, in late March.

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

Though Russia likely couldn’t capture Kharkiv without a massive buildup of troops and armor, 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 cause disruptions 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 20 miles south of the border.

Russia is 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. “Not all of our partners are currently fulfilling the agreements in a timely manner,” Zelenskyy said Friday in a post on X, though he didn’t specify which.

The Ukrainian army is on the defensive along the roughly 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 fall 2022. The bold counterattack helped persuade Western countries that Ukraine could defeat Russia on the battlefield and merited military support.

Meanwhile, Russian officials said a Ukrainian drone struck an oil refinery inside Russia on Friday. The drone hit a refinery near 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.