The latest report on Syria for the United Nations Human Rights Council says all sides are committing war crimes and crimes against humanity.
Government forces massacred civilians, shelled hospitals and used cluster bombs extensively between May and July, a commission of inquiry found.
Rebel fighters were increasingly guilty of summary executions, the report says.
The perpetrators of the crimes did not fear accountability and referral to justice was imperative, it adds.
The report comes a day before US Secretary of State John Kerry is due to meet Russian Foreign Minister Sergei Lavrov in Geneva to discuss a response to the use of chemical weapons on the outskirts of Damascus last month.
Hundreds of people were killed in an attack which the US, UK and France insist could only have been carried out by Syria's government. Syria strongly denies it was to blame.
'No military solution'
The Independent Commission of Inquiry on Syria was set up by the UN Human Rights Council in 2011 to monitor a conflict that has left more than 100,000 people dead.
Its investigators, who have been denied entry to Syria, based their latest report covering 15 May to 15 July on 258 interviews with eyewitnesses, video recordings and satellite imagery.
They found that Syria was a "battlefield", with millions of people displaced, cities and towns suffering "relentless shelling and sieges", and massacres being "perpetrated with impunity".
"Government forces have committed gross violations of human rights and the war crimes of torture, hostage-taking, murder, execution without due process, rape, attacking protected objects and pillage," the report says.
Hospitals have also been bombed, crops burnt, and water denied to desperate communities.
"Anti-government armed groups have committed war crimes, including murder, execution without due process, torture, hostage-taking and attacking protected objects. They have besieged and indiscriminately shelled civilian neighbourhoods," the report adds.
In one incident in June, a 15-year-old boy was executed by jihadist rebels, who accused him of blasphemy, in the northern city of Aleppo.
Rebel and Kurdish militia fighters have also been using child soldiers.
The report says that since the uprising against President Bashar al-Assad began in March 2011 there had been a total of at least eight massacres perpetrated by government forces and supporters, and one by rebel fighters. Nine further mass killings are being investigated.
The investigators say allegations were received regarding the use of chemical weapons, but that on the evidence currently available they were unable to identify the agents used or the perpetrators.
However, they say they now have a long list of individuals known to have committed war crimes and that they must be brought to justice.
"The perpetrators of these violations and crimes, on all sides, act in defiance of international law. They do not fear accountability. Referral to justice is imperative."
And, in a message to US and Russian diplomats preparing to meet in Geneva, the investigators insist: "There is no military solution to this conflict. Those who supply arms create but an illusion of victory."