A United Nations investigation has concluded the Syrian government or its allies was likely responsible for attacks on a school, a hospital, and two other civilian facilities, but the probe avoided specifically blaming Syria's main military sponsor, Russia, drawing rebukes from rights activists.

Human rights groups criticized the report, whose executive summary was released on April 6, as well as the United Nations for restricting the investigation to a narrow line of inquiry.

The inquiry produced "a "mealy-mouthed' report, all to avoid offending Russia, the prime offender along with Syria," Kenneth Roth, the head of Human Rights Watch, said on Twitter.

Rights groups have long pushed the world body to investigate alleged war crimes committed during the nine-year conflict, which has devastated Syria and killed hundreds of thousands.

The executive summary of the 185-page confidential report said that four civilian facilities -- a child-care facility, a hospital, a school, and a health-care center -- were targeted, and it was "highly probable" that the Syrian government's forces, or its allies, were responsible.

It was "plausible" that a separate attack on a second health center was also carried out by the Syrian government or its allies, the report found.

UN Secretary-General Antonio Guterres appointed a board of inquiry nearly two years ago, authorizing it to investigate incidents in Syria's northwest after Russia and Turkey agreed to establish a "de-escalation zone" in Idlib on September 2018.

At the time of the board's establishment, Russia called the war crimes accusations "a lie." Syria also disputed the allegations.

Rights groups have repeatedly accused Syrian government forces of committing wide-scale war crimes, purposely targeting civilians. Rights groups have also focused attention on Russia, which is Syria's main economic and military sponsor.

Russia's intervention, in September 2015, turned the tide of the war to the advantage of Bashar al-Assad's forces. The northwestern Idlib Province is currently the last rebel stronghold holding out.

Last year, The New York Times published an in-depth investigation of Syrian civilian facilities purposely being targeted.

The report, which included recordings of radio communications among Russian fighter pilots, directly incriminated Russia in attacks on civilian hospitals.