Russian forces were pressing ahead Friday with their offensive in north-east Ukraine as Russian President Vladimir Putin said there were no current plans to occupy Kharkiv city, the regional capital.
On a trip to China, Putin said the assault was direct retaliation for Ukrainian shelling of Russia's border regions and that Moscow was trying to create a "security zone".
"This is their fault because they have shelled and continue to shell residential neighbourhoods in border areas," Putin told reporters, adding there was no intention at this stage to take Kharkiv with its population of over one million about 40 kilometres from the front lines.
Moscow launched the surprise offensive into Ukraine's north east on May 10, sending thousands of troops across the border and unleashing artillery fire on several settlements, including the almost deserted town of Vovchansk.
Oleg Synegubov, governor of the Kharkiv region, said Russian forces were trying to surround Vovchansk, which had a pre-war population of around 18,000, and that Ukraine's forces were "resisting" the Russian onslaught.
"The enemy has actually started to destroy the city. It is not just dangerous to be there, but impossible," Synegubov said in a briefing.
But he warned Russia was also gaining ground near Lukyantsi, a village much further west that Kyiv pulled back from earlier this week amid heavy fire.
Ukraine army chief Oleksandr Syrsky said Russia was trying to force Ukraine to pull troops from its reserves.
"We realise that there will be heavy fighting ahead and the enemy is preparing for it," he said.
Ukraine has evacuated almost 9,000 people in the week since Russian forces stormed across the border last Friday.
- Drone wave -
Putin's warning came hours after Ukraine launched a wave of drones at Russia and the annexed Crimea peninsula overnight, killing two people including a child and setting oil infrastructure ablaze.
The attack was Ukraine's largest aerial offensive in weeks and one of many to target Russian energy facilities, which Kyiv says Moscow uses to fuel the war.
The Russian military said it had intercepted or downed more than 100 Ukrainian drones in the south of the country, Crimea and Black Sea during the night.
However, officials in multiple regions reported destruction.
One drone struck a family driving near the border in the Belgorod region, killing a mother and her four-year-old son, the region's governor Vyacheslav Gladkov said.
"The child was in a critical condition. Doctors did everything possible to save him. (But) to much grief, the four-year-old died in hospital," he said.
In the coastal town of Tuapse in the southern Krasnodar region, Ukrainian drones hit an oil refinery for the second time this year, sparking a large fire that was later put out, authorities said.
And several fires also erupted after a drone attack on Novorossiysk, another coastal city in the Krasnodar region, local governor Veniamin Kondratyev said.
A source in Ukraine's defence sector confirmed Kyiv had targeted oil facilities in both cities, and had also hit an electrical substation in the Russian-controlled port of Sevastopol.
The city's Russian-installed governor Mikhail Razvozhayev said there had been a "partial blackout" after debris from downed drones damaged a substation, and that work to restore power was ongoing.