They blamed both government forces and armed rebels for carrying out executions and torture, and spreading terror among civilians.
The investigators' latest report – covering the six months to mid-January – was based on 445 interviews conducted abroad with victims and witnesses, as they have not been allowed into Syria.
The team, led by Brazilian Paulo Pinheiro, called on the UN Security Council to ensure accountability for grave violations, by referring the suspects to the International Criminal Court (ICC) at the Hague, in Holland.
Karen Konig AbuZayd – one of the four commissioners on the team of two dozen experts who drew up the 131-page report – said: "We have information suggesting people who have given instructions and are responsible for government policy. People who are in the leadership of the military, for example."
But the list of suspects remains secret. It will be entrusted to UN High Commissioner for Human Rights, Navi Pillay, upon expiry of the team's current mandate at the end of March.
Ms Pillay, a former judge at the ICC, said at the weekend Syrian President Bashar al-Assad should be probed for war crimes and called for action by the international community, including possible military intervention.
Last week she appealed to the Security Council to refer Syria to the ICC, claiming the death toll in the nearly two-year-old conflict was approaching 70,000 people.
Government forces have carried out shelling and aerial bombardment across Syria – including Aleppo, Damascus, Deraa, Homs and Idlib – the UN investigators said, citing corroborating evidence gathered from satellite images.
"In some incidents, such as in the assault on Harak, indiscriminate shelling was followed by ground operations during which government forces perpetrated mass killing," they added, referring to a town in the southern province of Deraa where it is claimed 500 civilians were killed in August.
The report continued: "Government forces and affiliated militias have committed extra-judicial executions, breaching international human rights law. This conduct also constitutes the war crime of murder.
"Where murder was committed as part of a widespread or systematic attack against a civilian population, with knowledge of that attack, it is a crime against humanity. Syrian armed forces have implemented a strategy that uses shelling and sniper fire to kill, maim, wound and terrorise the civilian inhabitants of areas that have fallen under anti-government control."
Government forces had used cluster bombs, it claimed, but no evidence was found of either side using chemical weapons.
Rebel forces fighting to topple Mr Assad have also committed war crimes – including murder, torture, hostage-taking and using children under the age of 15 in hostilities – the UN report said, adding rebel snipers had caused "considerable civilian casualties".
However, the report added: "The violations and abuses committed by anti-government armed groups did not reach the intensity and scale of those committed by government forces and affiliated militia."