GENEVA — As the United States and Russia searched for a diplomatic solution to the crisis over Syria’s chemical weapons, a four-person United Nations rights panel presented detailed evidence on Wednesday of what it said were war crimes and crimes against humanity committed by pro-government forces and, to a lesser extent, rebels in the 30-month-old conflict.

Bolstered by weapons and money from regional and global powers waging a proxy war, Syria’s government and rebel forces have committed murder, torture, rape and indiscriminate attacks on civilians, without fear of future punishment, the panel, a Commission of Inquiry that was expanded last fall, said in its latest report, to be presented to the United Nations Human Rights Council here in Geneva on Monday.

The report was careful to hold both sides responsible, but the unevenness of the conflict — with heavily armed government forces battling rebels with scanty, sometimes homemade arsenals — was evident. Of the nine mass killings the panel investigated for the report, eight were attributed to the government side and one to rebels.

“Relentless shelling has killed thousands of civilians and displaced the populations of entire towns,” the report said, leaving government responsibility implicit. “Massacres and other unlawful killings are perpetrated with impunity. An untold number of men, children and women have disappeared. Many have died in detention.”