At least 20 people have died after Russian forces shelled a civilian evacuation convoy in the north-east of Ukraine, a senior official said.
Bombardments have intensified as Moscow illegally annexed a swath of Ukrainian territory in a sharp escalation of the war.
Kharkiv region governor Oleh Syniehubov said the convoy was struck in the Kupiansy district on Saturday, calling the attack on people who were trying to flee the area to avoid being shelled “сruelty that can’t be justified”.
Russian forces have not acknowledged or commented on the attack, apparently the second in two days to hit a humanitarian convoy.
Russian troops have retreated from much of the Kharkiv region after a successful Ukrainian counter-offensive last month but continued to shell the area.
The attack comes at a pivotal moment in Russian President Vladimir Putin’s war.
Facing a Ukrainian counter-offensive, Mr Putin this week heightened threats of nuclear force and used his most aggressive, anti-Western rhetoric to date.
Ukrainian President Volodymyr Zelensky and his military vowed to keep fighting to liberate the annexed regions and other Russian-occupied areas.
Ukrainian officials said on Saturday their forces had surrounded thousands of Russian soldiers holding the strategic eastern city of Lyman, which is located in one of the four incorporated areas.
Mr Zelensky formally applied on Friday for Ukraine to join Nato, increasing pressure on Western allies to help defend the country.
Also on Saturday, Ukraine’s nuclear power provider said Russian forces blindfolded and detained the head of Europe’s largest nuclear plant.
Soldiers seized the director-general of the Zaporizhzhia nuclear power plant, Ihor Murashov, at about 4pm on Friday, Ukrainian state nuclear company Energoatom said.
The incident was just hours after Mr Putin signed treaties to absorb Moscow-controlled Ukrainian territory into Russia, including the area around the nuclear plant.
Amid growing international sanctions and condemnation of Russia, a Ukrainian counter-offensive that has embarrassed the Kremlin appeared on the verge of retaking more ground.
A Ukrainian official said on Saturday that the Russian-occupied city of Lyman was surrounded, with some 5,000 Russian forces trapped there.
The Institute for the Study of War, a Washington-based think tank, said Ukraine likely will retake Lyman in the coming days.
Citing Russian reports, the institute said it appeared Russian forces were retreating from Lyman, some 100 miles south-east of Kharkiv, Ukraine’s second-largest city.
That corresponds to online videos purportedly showing some Russian forces falling back as a Ukrainian soldier said they had reached Lyman’s outskirts.
It said Ukraine also was making “incremental” gains around Kupiansk and the eastern bank of the Oskil River, which became a key front line since the Ukrainian counteroffensive regained control of the Kharkiv region in September.
The Russian army struck the southern Ukrainian city of Mykolaiv twice overnight, once with drones and the second time with missiles, according to regional governor Vitaliy Kim.