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, the mission said in a report on Wednesday.

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

The OSCE is an international organisation that includes former Cold War foes the United States and Russia and various countries in Europe, Central Asia and North America.

"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.

Not all violations of international humanitarian law are war crimes. The mission comprised three professors of international law from Austria, Switzerland and the Czech Republic.

Russia's mission to the OSCE said on Twitter the report "is based solely on unfounded propaganda theses, contains references to dubious sources and logical stretches in the style of 'highly likely'".

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 March 16 attack on Mariupol's Drama Theatre, in which local Ukrainian officials said roughly 300 people were 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 what it called 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".

"Taken as a whole, the report documents the catalog of inhumanity perpetrated by Russia's forces in Ukraine," U.S. ambassador to the OSCE Michael Carpenter said in a statement.

"This includes evidence of direct targeting of civilians, attacks on medical facilities, rape, executions, looting, and forced deportation of civilians to Russia."