Ukraine has labelled Russia a "terrorist state" after Moscow's UK embassy tweeted that Ukrainian Azov battalion soldiers deserved a "humiliating death" by hanging.
The embassy tweet came after more than 50 Ukrainian prisoners of war (POWs) held by Russia were killed - Azov soldiers reportedly among them.
They died in an attack on Olenivka prison in Russian-held eastern Ukraine.
Ukraine and Russia blame each other for the attack that killed the prisoners.
Twitter did not remove the tweet from the Russian Embassy in the UK, but said it broke Twitter anti-hate rules.
Besides the Ukrainian government, many other Twitter users voiced outrage at the tweet. Twitter said it may be in the public interest to keep the post accessible.
The Olenivka prison camp is controlled by the Russian-backed self-proclaimed Donetsk People's Republic (DNR).
What happened there on Friday remains unclear. Unverified Russian video footage of the aftermath shows a tangle of wrecked bunk beds and badly charred bodies.
After the attack the Russian embassy in the UK tweeted that Azov "militants deserve execution, but death not by firing squad but by hanging, because they're not real soldiers. They deserve a humiliating death".
The tweet included a video clip showing a couple in a wrecked building, accusing Azov troops of having shelled their home. The embassy's call for execution repeats what the man in the video says.
Azov troops were captured in May after fiercely defending Azovstal, a giant steelworks in the southern city of Mariupol. Russia has long accused the battalion of being neo-Nazis and war criminals.
Andriy Yermak, Ukrainian presidential chief of staff, wrote on Telegram that "Russia is a terrorist country".
"In the 21st Century, only savages and terrorists can say at a diplomatic level that people deserve to be executed by hanging for nothing. The RF [Russia] is a state spUkraine has called for the United Nations and Red Cross to be allowed to investigate the deaths at Olenivka. The Red Cross says it is seeking access to the prison to help with evacuating and treating the wounded. onsor of terrorism. What more proof do you need?" he said.
Ukraine says the site was targeted by Russia in an effort to destroy evidence of torture and killing. President Volodymyr Zelensky described the incident as a "deliberate Russian war crime".
Ukraine's foreign ministry spokesman Oleg Nikolenko tweeted: "There is no difference between Russian diplomats calling for execution of Ukrainian prisoners of war and Russian troops doing it in Olenivka.
"They are all accomplices in these war crimes and must be held accountable."
DNR spokesman Daniil Bezsonov said the strike had killed 53 people and wounded 75. He called it a "direct hit on a barracks holding prisoners".