KYIV, Ukraine (AP) — U.N. human rights investigators say Ukrainian prisoners of war appear to be facing “systematic” mistreatment — including torture — both when they are captured and when they are transferred into areas controlled by Russian forces or Russia itself.

The head of a monitoring mission set up by the U.N. human rights office said Tuesday that Russia must address such mistreatment, which amounts a “grave violation” of international law.

The mission issued its first comprehensive look at rights violations and abuses committed by both sides of the war between Feb. 1 and July 31 — covering the first months after Russian forces invaded Ukraine on Feb. 24. The mission, which tracks the situation daily, has been monitoring rights in Ukraine ever since a conflict involving Russian-backed insurgents began in eastern Ukraine in 2014.

The report, based on a strict methodology to verify claims, documented crimes including enforced disappearances, arbitrary detention, summary executions, torture, and sexual violence — much of which have been brought to light by rights monitors, advocacy groups and the news media in recent months.

The team hopes to chronicle rights abuses and violations in detail, in hopes that perpetrators can be held to account one day.

On prisoners of war, the team said it received “unimpeded access” to places of internment in areas controlled by Ukraine’s government — but not in Russia or areas controlled by Russian forces or their affiliates.

In the cases it documented, the mission found that the “vast majority” of Ukrainian prisoners of war were subjected to “torture or cruel and degrading treatment by the detaining power,” a statement from the mission said.

Matilda Bogner, the mission’s chief, said such mistreatment of Ukrainian POWs “appears to be systematic, not only upon their capture, but also following their transfer to places of internment” both in areas of Russian-controlled Ukraine and Russia itself.

The team also found that some Russian POWs had been subjected to torture and ill-treatment, mostly during capture or during transit to places of internment.

Bogner called for investigations of all allegations of violations of international law, and prosecution as warranted, in connection with the situation in Ukraine.