Russia and its proxy forces in Ukraine are operating 21 locations used to detain, interrogate and process prisoners of war and civilians, according to a new report by Yale University researchers backed by the U.S. State Department as part of efforts to hold Moscow accountable.

The report, seen by Reuters ahead of its publication on Thursday, cites commercial satellite imagery and open-source information to identify with "high confidence" the separate locations - including facilities that previously served as schools, markets and regular prisons. It also identifies possible graves at one prison complex.

The Humanitarian Research Lab at Yale School of Public Health that produced the report is a partner in a U.S. State Department-funded Conflict Observatory launched in May to capture and analyze evidence of war crimes and other atrocities allegedly perpetrated by Russia in Ukraine.

Nathaniel Raymond, the lab's executive director, said the findings showed Russia and its proxies had established a "system of filtration" to sort people in areas that fall under Russian occupation that represents a "human rights emergency."

Reports of abuses had already emerged from the sites, including at a prison complex near Olenivka, where 53 Ukrainian prisoners of war were allegedly killed in a blast there on July 29.

Yale researchers newly identified disturbances in the earth consistent with individual or mass graves as early as April, the report said, matching the account of a former prisoner who reported that detainees were forced to dig graves at around that time.

Though the researchers did not reach any conclusion on the fate of the Ukrainian prisoners of war at the prison, they also confirmed further disturbances elsewhere in the compound were captured on July 27, before the blast at Olenivka. The New York Times has previously reported disturbances at the complex in July.

"Conditions are absolutely ripe for extreme abuse and in many cases, as we've seen in Olenivka, we see indications that we may have a five-alarm fire," Raymond said, adding it was not known how many civilians had passed through or were still being held at the sites.

Ukrainian officials have accused Russia of deporting hundreds of thousands of people from Russian-occupied areas of Ukraine.

Moscow has denied intentionally targeting civilians since invading Ukraine on Feb. 24 in what it calls a "special military operation" and says it is offering humanitarian aid to those wanting to leave.

The Russian Embassy in Washington said in July that U.S. allegations about the detention of Ukrainians in occupied areas were an attempt to stoke "Russophobia" and vilify Russian armed forces.

'ACCOUNTABILITY IS IMPERATIVE'

Thursday's report focused on Donetsk region, where Russia and its proxy the self-declared Donetsk People's Republic took control of most of the city of Mariupol in March. The city's mayor said in April that about 40,000 civilians from the city had been forcibly moved into Russian-controlled territory or taken to Russia.

The report identified a system that brings in civilians in conflict-affected areas, puts them through registration and interrogation before they are either released, kept in detention, or transported to Russia.

Researchers verified the 21 locations with at least five independent sources and believe at least another seven sites are part of the filtration system and could be verified at a later date, Raymond said.

The U.S. National Intelligence Council in June said it had identified 18 possible locations used for filtration in Ukraine and western Russia.

The State Department on Thursday in a statement again called for Russia to halt all filtration operations and forced deportations and to provide outside observers access.

"President Putin and his government will not be able to engage in these persistent abuses with impunity. Accountability is imperative, and the United States and our partners will not be silent," the statement read.