Russia is facing mounting accusations of war crimes in Ukraine on Friday, including forcing thousands of people into interrogation camps as the number of refugees reported to have fled the conflict surpassed 6 million.

The Russian invasion has also led to a seismic policy change by Finland, whose leaders said Thursday the previously neutral nation must apply to join NATO "without delay" – triggering a blunt warning of retaliation from the Kremlin.

Throughout the 11-week conflict, Russian forces have been accused of committing atrocities – including the killing of unarmed civilians, torture and rape.

CNN and the BBC on Thursday released what they said was security camera footage showing Russian soldiers with assault rifles shooting two Ukrainian civilians in the back.

The two men appeared unarmed – the footage showed the soldiers frisking them before allowing them to walk away on the premises of a business on the outskirts of the capital Kyiv. One man died on the spot, the other shortly after, according to the outlets. The killings took place on March 16 and are being investigated as a war crime, CNN said.

The U.N. Human Rights Council voted 33-2 on Thursday to investigate alleged atrocities by Russian troops in Ukraine. Ukrainian prosecutors say they have received reports of more than 10,000 alleged crimes.

'Brutal interrogations'

The United Nations High Commissioner for Refugees (UNHCR) said Thursday more than 6 million people had fled Ukraine, more than half of them going to neighboring Poland.

Women and children make up 90% of the refugees, the UNHCR said.

The United States on Thursday accused Russia of forcibly taking tens of thousands of Ukrainians to "filtration camps" in Russia or Russian-controlled territory where they are subjected to "brutal interrogations."

"These actions amount to war crimes," said Michael Carpenter, the U.S. ambassador to the Organization for Security and Cooperation in Europe (OSCE).

"We must not allow this evil to stand."

The remarks backed Kyiv's allegation that 1.2 million people have been taken to Russia or Russian-controlled areas.

Fighting in Ukraine has been concentrated in the south and east since Russia abandoned attempts to seize the capital.

Ukraine's presidency said shelling continued throughout Lugansk – part of the Donbass region where its forces are fiercely opposing Russian troops and Kremlin-backed separatists.

Ukrainian President Volodymyr Zelenskyy said Thursday that Russian forces had destroyed 570 health care facilities.

"What for? It's nonsense. It's barbarity," he said.

In the northeastern region of Chernigiv, three people were killed and 12 others wounded early Thursday in a strike on a school in Novgorod-Siversky, the emergency services said.

In the southern port city of Mariupol, troops at the Azovstal steelworks have been holding out against Russian bombardment for weeks, refusing demands to surrender.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said "difficult talks" were underway on the evacuation of 38 seriously wounded soldiers.

Russia's army said it struck Donetsk and Kharkiv on Thursday, killing more than 170 people and destroying Ukrainian drones and rockets.

First war crimes trial

A Ukrainian court held a preliminary hearing on Friday in the first war crimes trial arising from Russia's Feb. 24 invasion, after charging a captured Russian soldier with the murder of a 62-year-old civilian.

The case is of huge symbolic importance for Ukraine. The Kyiv government has accused Russia of atrocities and brutality against civilians during the invasion and said it has identified more than 10,000 possible war crimes.

Russia has denied targeting civilians or involvement in war crimes and accused Kyiv of staging them to smear the reputation of its forces. The Kremlin told reporters on Friday that it had no information about a war crimes trial.

The defendant told the court he was Vadim Shishimarin, born in Russia's Irkutsk region and confirmed that he was a Russian serviceman in the short, preliminary hearing. The court will reconvene on May 18, the judge said.

The Kyiv district court's website said Shishimarin was accused of "violations of the laws and norms of war."

He will tell the court at a later date whether or not he denies the charge, his lawyer Viktor Ovsyannikov said.

Shaven-headed and looking scared, Shishimarin wore a casual blue and grey hoodie and was led into the courtroom by police to a glass booth for defendants.

The Ukrainian prosecutor general's office said the defendant was a 21-year-old tank commander in the Kantemirovskaya tank division from the Moscow region. The prosecutor general had published a photograph of him ahead of the hearing.

If convicted he faces up to life imprisonment over the killing in the northeast Ukrainian village of Chupakhivka, east of the capital Kyiv, on Feb. 28.

Many more cases expected

In the courthouse, Shishimarin was questioned by a judge who addressed him in Ukrainian and in Russian. He had an interpreter with him. Reuters could not reach him or his legal representative for comment ahead of the hearing.

State prosecutor Andriy Synyuk told reporters after the hearing: "This is the first case today. But soon there will be a lot of these cases."

Michelle Bachelet, the U.N. high commissioner for human rights, said on Thursday there were many examples of possible war crimes since the Russian invasion and that 1,000 bodies had been recovered so far in the Kyiv region.

The International Criminal Court (ICC) said on April 25 that it would take part in a joint team with Ukrainian, Polish and Lithuanian prosecutors investigating war crimes allegations against Russian forces.

Russia calls its actions in Ukraine a "special operation" to disarm the country and protect it from fascists, denying its forces committed abuses. Kyiv and its Western backers say the fascism claim is a false pretext for an unprovoked war of aggression.