KYIV – Thousands more civilians have fled Russia’s renewed ground offensive in Ukraine’s northeast that has targeted towns and villages with a barrage of artillery and mortar fire, officials said Sunday.

The intense battles have forced at least one Ukrainian unit to withdraw in the Kharkiv region, capitulating more land to Russian forces across less defended settlements in the so-called contested “gray zone” along the Russian border.

By Sunday afternoon, the town of Vovchansk, with a prewar population of 17,000, emerged as a focal point in the battle. Volodymyr Tymoshko, the head of the Kharkiv regional police, said Russian forces were in the outskirts of the town and approaching from three directions. A Russian tank was spotted along a major road leading to the town, Tymoshko said, illustrating Moscow’s confidence to deploy heavy weaponry.

An Associated Press team, positioned in a nearby village, saw plumes of smoke rising from the town as Russian forces hurled shells. Evacuation teams worked non-stop throughout the day to take residents, most of whom were elderly, out of harm’s way.

At least 4,000 civilians have fled the Kharkiv region since Friday, when Moscow’s forces launched the operation, Gov. Oleh Syniehubov said in a social media statement. Heavy fighting raged Sunday along the northeast front line, where Russian forces attacked 27 settlements in the past 24 hours, he said.

Analysts say the Russian push is designed to exploit ammunition shortages before promised Western supplies can reach the front line. Ukrainian soldiers said the Kremlin is using the usual Russian tactic by launching a disproportionate amount of fire and infantry assaults to exhaust their troops and firepower. By intensifying battles in what was previously a static patch of the front line, Russian forces threaten to pin Ukrainian forces in the northeast, while carrying out intense battles farther south where Moscow is also gaining ground.

It comes after Russia stepped up attacks in March targeting energy infrastructure and settlements, which analysts predicted were a concerted effort to shape conditions for an offensive.

Meanwhile, a 10-story apartment block collapsed in the Russian city of Belgorod, near the border, injuring at least 19 people. Russian authorities said the building collapsed following Ukrainian shelling. Ukraine has not commented on the incident.

Ukrainian President Volodymyr Zelenskyy said that halting Russia’s offensive in the northeast was a priority, and that Kyiv’s troops were continuing counteroffensive operations in seven villages around the Kharkiv region.

“Disrupting the Russian offensive intentions is our number one task now. Whether we succeed in that task depends on every soldier, every sergeant, every officer,” Zelenskyy said.

The Russian Defense Ministry said Saturday that Moscow’s forces had captured five villages on the border of Ukraine’s Kharkiv region and Russia. These areas were likely poorly fortified due to the dynamic fighting and constant heavy shelling, easing a Russian advance.

Ukraine’s leadership has not confirmed Moscow’s gains.

A Ukrainian unit said that they had been forced to retreat in some areas and that Russian forces had captured at least one more village late Saturday. Tymoshko said Russian tactics in Vovchansk mirrored those used in the battles for Bakhmut and Avdiivka in the Donetsk region in which heavy aerial attacks were accompanied by droves of infantry assaults.

In a video Saturday evening, the Hostri Kartuzy unit, part of the special forces’ detachment of Ukraine’s national guard, said that they were fighting for control of the village of Hlyboke.

“Today, during heavy fighting, our defenders were forced to withdraw from a few more of their positions, and today, another settlement has come completely under Russian control. As of 20:00, fighting for the village of Hlyboke is ongoing,” the fighters said in the clip.

The U.S.-based Institute for the Study of War said Saturday that it believed claims that Moscow had captured Strilecha, Pylna, Pletenivka and Borsivika were accurate, and that geolocated footage also appeared to show that Russian forces have seized Morokhovets and Oliinykove. It described the recent Russian gains as “tactically significant.”

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