Thousands of civilians have been evacuated from the Kharkiv region this weekend after Moscow's forces launched a renewed ground offensive in Ukraine.
Russia has targeted towns and villages in north-east Ukraine with a barrage of artillery and mortar fire.
At least 4,000 members of the public have fled the Kharkiv area, Governor Oleh Syniehubov said in a social media statement.
Heavy fighting raged on Sunday along the front line, where Russian forces attacked 27 settlements in the past 24 hours, he said.
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 "grey zone" along the Russian border.
Meanwhile, a 10-storey apartment block collapsed in the Russian city of Belgorod, near the border, with several deaths and injuries reported. Russian authorities said the building collapsed following Ukrainian shelling. Ukraine has not commented on the incident.
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.
It comes after Russia stepped up attacks in March targeting energy infrastructure and settlements, which analysts predicted were a concerted effort by Moscow to shape conditions for an offensive.
Ukrainian President Volodymyr Zelensky said disrupting Russia's offensive in the area is a priority, and that Kyiv's troops are continuing counter-offensive operations in seven villages around the Kharkiv region.
"Disrupting the Russian offensive intentions is our number one task now," he said. "Whether we succeed in that task depends on every soldier, every sergeant, every officer."
The Russian Defence Ministry said on Saturday that Moscow's forces had captured five villages on the border of Ukraine's Kharkiv region and Russia. These areas were likely to have been 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 it had been forced to retreat in some areas and Russian forces had captured at least one more village late on Saturday.
In a video on Saturday evening, the Hostri Kartuzy unit, part of the special forces' detachment of Ukraine's national guard, said it was 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 2000, fighting for the village of Hlyboke is ongoing," the fighters said in the clip.
In the war's early days, Russia made a botched attempt to quickly storm Kharkiv but retreated from its outskirts after about a month.
Seven months later, Ukraine's army pushed them out of Kharkiv. The bold counter-attack helped persuade Western countries that Ukraine could defeat Russia on the battlefield and merited military support.
