The European Union has demanded that all war criminals face appropriate punishment after new mass graves were found in Ukraine.
Jan Lipavsky, Foreign Minister of the Czech Republic, which currently holds the EU's rotating presidency, also called for the establishment of an international tribunal for war crimes after the discovery on the outskirts of Izium.
"In the 21st century, such attacks against the civilian population are unthinkable and abhorrent," he said in a Twitter message.
"We must not overlook it. We stand for the punishment of all war criminals.
"I call for the speedy establishment of a special international tribunal that will prosecute the crime of aggression."
The appeal referred to the Ukrainian authorities' discovery of about 450 graves outside the formerly Russian-occupied city in the eastern Kharkiv region.
Ukrainian President Volodymyr Zelenskyy in his evening address on Saturday said "new evidence of torture" had been presented by some of the bodies exhumed.
He said more than 10 torture chambers had been found in cities and towns liberated from Russian occupation in the Kharkiv region, where Ukraine's military made sweeping gains over the past week.
"That's what the Nazis did," Mr Zelenskyy said. "This is what Ruscists do. And they will be held accountable in the same way — both on the battlefield and in courtrooms.
Oleg Synegubov, head of Kharkiv's regional administration, said that of the bodies exhumed on Saturday, "99 per cent showed signs of violent death".
"There are several bodies with their hands tied behind their backs, and one person is buried with a rope around his neck," he said on social media.
The EU is "deeply shocked" at the newest discovery of a mass grave left by the Russians in the nearly seven-month war, said the bloc's foreign policy chief Josep Borrell.
Mr Borrell called for an immediate stop to the "inhuman behaviour by the Russian forces, in total disregard of international humanitarian law and the Geneva Conventions".
French President Emmanuel Macron described what had happened in Izium as atrocities.
Those responsible "will have to answer for their acts. There is no peace without justice," he said on Twitter.
US Secretary of State Antony Blinken said the mass graves likely provided more evidence that Russia was committing war crimes in its pro-Western neighbour.
The Ukrainian Parliament's human rights commissioner, Dmytro Lubinets, said there were "probably more than 1,000 Ukrainian citizens tortured and killed in the liberated territories of the Kharkiv region".
The United Nations in Geneva has said it hopes to send a team to determine the circumstances of the deaths.
The macabre discoveries came a little more than five months after the Russian army, driven out of Bucha near the capital Kyiv, left behind hundreds of corpses of civilians, many of whom had signs of torture and summary executions.
On Thursday, President of the European Commission, Ursula von der Leyen, said she wanted Russian President Vladimir Putin to face the International Criminal Court over war crimes in Ukraine.
Ukrainian forces have recaptured thousands of square kilometres in recent weeks thanks to a counter-offensive in the north-east and they are now focused on Russian positions in the south, as the fighting and bombings continue.
In the north-eastern town of Kupiansk, which was recaptured last week by Ukrainian forces, clashes continued with the Russian army entrenched on the eastern side of the Oskil river.
A column of smoke rose over the east of the city, where an ammunition depot was burning.
In a statement on Saturday, the Ukrainian army said "the enemy carried out four missile strikes and 15 air strikes during the day, as well as more than 20 multiple rocket launcher strikes on civilian and military sites in Ukraine".
In its daily briefing in Moscow, the Kremlin said it had carried out "high-precision" strikes against Ukrainian positions in the Mykolaiv and Kharkiv regions.
Meanwhile, Ukraine's Zaporizhzhia nuclear plant has begun receiving power from the national grid once again, the UN's atomic agency (IAEA) said on Saturday, after it was cut off from external power, raising the chances of an accident.
The Russian-occupied plant, the largest in Europe, had been cut from the power grid since September due to bombardment. Russia and Ukraine accuse each other of carrying out the attacks.
— With reporting from AFP