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.

The Herald: President Vladimir Putin has reissued threats of nuclear force (Grigory Sysoyev/Sputnik/Government Pool Photo/AP/PA)President Vladimir Putin has reissued threats of nuclear force (Grigory Sysoyev/Sputnik/Government Pool Photo/AP/PA) (Image: PA)

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.