Many Ukrainians have reportedly been mistreated in filtration camps set up by Russia, according to the British defence ministry.
Its latest assessment of the war estimated more than 2.5m have been evacuated to Russia from Ukraine.
“Russia continues to face accusations that it is forcibly deporting Ukrainians,” the UK’s defence ministry said.
It also said Russian forces were slowly advancing westwards towards the town of Siversk in the Donetsk region.
It came after Volodymyr Zelensky called Russia a “terrorist state” after a missile strike in Ukrainian city Vinnytsia killed at least 23 people and injured dozens.
“This day once again proved that Russia must be officially recognized as a terrorist state. No other state in the world poses such a terrorist threat as Russia,” the Ukrainian president said.
In other news, Ukraine’s allies, including the US and more than 40 other countries, agreed on Thursday to coordinate investigations into Russia-led suspected war crimes.