A mission of experts set up by Organization for Security and Cooperation and Europe (OSCE) nations has found evidence of war crimes and crimes against humanity by Russia in Ukraine, an initial report by the mission said on Wednesday.

The mission was set up last month by 45 of the OSCE's 57 participating countries to look into possible offences, including war crimes in Ukraine, and to pass on information to bodies such as international tribunals. Russia opposed it.

"The mission found clear patterns of IHL (international humanitarian law) violations by the Russian forces," the report said, citing failures to take necessary precautions, act proportionately or spare sites like schools and hospitals.

Despite Russian denials, the report said a March 9 attack on the Mariupol Maternity House and Children's Hospital was carried out by Russia and those responsible had committed a war crime.

It also said the attack on Mariupol's Drama Theatre on March 16, in which local Ukrainian officials say many as 300 people may have been killed, was a war crime.

"The Mission is not able to conclude whether the Russian attack on Ukraine per se may qualify as a widespread or systematic attack directed against a civilian population," it said, referring to the context in which crimes like murder and rape constitute crimes against humanity.

"It however holds that some patterns of violent acts violating IHRL (international human rights law), which have been repeatedly documented in the course of the conflict, such as targeted killing, enforced disappearance or abductions of civilians ... are likely to meet this qualification," it said.

"Any single violent act of this type, committed as part of such an attack and with the knowledge of it, would then constitute a crime against humanity."

The mission also found some "violations" by Ukraine, particularly in its treatment of prisoners of war, but it said Russia's violations "are by far larger in nature and scale".