War crimes including rape, torture, executions and confinement of children were committed by Russia in areas it occupied in Ukraine, the head of a UN-mandated investigation body said on Friday.

The commission is one of the first international bodies to reach the conclusion on the basis of field evidence. Ukraine and its Western allies have accused Russian soldiers of a litany of abuses since the Feb 24 invasion, but Moscow has regularly dismissed the allegations as a smear campaign.

"Based on the evidence gathered by the Commission, it has concluded that war crimes have been committed in Ukraine," Erik Mose, who heads the Commission of Inquiry on Ukraine, told the UN Human Rights Council in Geneva.

He did not give an estimate of how many crimes had taken place but said in a later interview that "a large number" had been committed by Russia and only two cases by Ukraine involving the ill-treatment of Russian soldiers.

Russia denies deliberately attacking civilians during what it calls its "special military operation".

Russia was called on to respond to the accusations at the council meeting but its seat was left empty. There was no immediate official reaction from Moscow.

The Ukrainian prosecutor's office did not immediately respond to a request for comment.

Investigators from the commission, created by the rights council in March, visited 27 places and interviewed more than 150 victims and witnesses in areas of Kyiv, Chernihiv, Kharkiv and Sumy formerly held by Russia.

They found evidence of a large number of executions including bodies with tied hands, slit throats and gunshot wounds to the head, Mose said.

He said investigators had identified victims of sexual violence aged between four and 82. While some Russian soldiers had used sexual violence as a strategy, the commission "has not established any general pattern to that effect", Mose added.

In reaction to the speech, several countries suggested that the abuses amouted to crimes against humanity. "So far we have not concluded that there are crimes against humanity," Mose said, adding that these were preliminary findings and evidence-gathering and analysis continued.

The commission will next turn its attention to allegations of "filtration" camps in Russian-occupied areas for processing Ukrainian prisoners as well as claims of forced transfers of people and adoption of Ukrainian children in Russia.

Ukraine and some other nations urged the commission to also investigate a mass burial site near the city of Izyum, in eastern Ukraine, where hundreds of bodies have been found.

"If left unanswered, (Russia's violations) will drag us into a dark world of impunity and permissiveness," Ukraine's envoy Anton Korynevych told the council by video link. Mose later said they intended to investigate the Izium graves.

Sometimes probes launched by the council can be used before national and international courts, such as in the case of a former Syrian intelligence officer jailed for state-backed torture in Germany in January.

Mose said he was in touch with the International Criminal Court about the commission's findings. The body is due to submit a complete report to the council at the end of its mandate in March 2023, including a possible list of perpetrators and recommendations on how to hold them to account.