Both government forces and armed rebels are committing war crimes, including killings and torture, spreading terror among civilians in a nearly two-year-old conflict, they said.
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 independent team, led by Brazilian Paulo Pinheiro, called on the UN Security Council to "act urgently to ensure accountability" for grave violations, possibly by referring the violators to the International Criminal Court for prosecution.
"The ICC is the appropriate institution for the fight against impunity in Syria. As an established, broadly supported structure, it could immediately initiate investigations against authors of serious crimes in Syria," the 131-page report said.
It added: "Individuals may also bear criminal responsibility for perpetuating the crimes identified in the present report. Where possible, individuals in leadership positions who may be responsible were identified alongside those who physically carried out the acts."
Karen Konig AbuZayd, one of the four commissioners on the team of some two dozen experts, told Reuters: "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."
"It is the first time we have mentioned the ICC directly. The Security Council needs to come together and decide whether or not to refer the case to the ICC. I am not optimistic."
But its third list of suspects, building on lists drawn up in the past year, remains secret. It will be entrusted to UN High Commissioner for Human Rights, Navi Pillay, upon expiry of its current mandate at the end of March, the report said.
Pillay, a former judge at the ICC, said on Saturday that Syrian President Bashar Assad should be probed for war crimes and called for immediate action by the international community, including possible military intervention.
"The evidence collected sits in the safe in the office of the High Commissioner against the day it might be referred to a court and evidence would be examined by a prosecutor," said a European diplomat.
The death toll in Syria is likely approaching 70,000 people, Pillay told the Security Council last week in a fresh appeal for it to refer Syria to the ICC, the Hague-based war crimes court.
Government forces have carried out shelling and aerial bombardment across Syria including Aleppo, Damascus, Deraa, Homs and Idlib, the independent 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," it said, referring to a town in the southern province of Deraa where residents told them that 500 civilians were killed in August.'Spreading terror'
"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," the UN report said.
They have targeted queues at bakeries and funeral processions, in violence aimed at "spreading terror among the civilian population", it said.
"Syrian armed forces have implemented a strategy that uses shelling and sniper fire to kill, maim, wound and terrorize the civilian inhabitants of areas that have fallen under anti-government armed group control," the report said.
Government forces had used cluster bombs, it said, but it found no credible evidence of either side using chemical arms.
Rebel forces fighting to topple Assad in the protracted and increasingly sectarian conflict have committed war crimes include murder, torture, hostage-taking and using children under age 15 in hostilities, the UN report said.
"They continue to endanger the civilian population by positioning military objectives inside civilian areas," it said. Rebel snipers had caused "considerable civilian casualties".
"The violations and abuses committed by anti-government armed groups did not, however, reach the intensity and scale of those committed by government forces and affiliated militia."
Foreign fighters, many of them from Libya, Tunisia, Saudi Arabia, Lebanon, Iraq and Egypt, have radicalized the rebels and helped detonate deadly improvised explosive devices, it said.
The two other commissioners are former chief ICC prosecutor Carla del Ponte and Vitit Muntarbhorn of Thailand.
"It is an investigative mechanism and its evidence can be given to relevant judicial authorities when the time comes. In the interim, it is the one piece of UN-approved machinery shining a light on abuses," the European diplomat said.
Referring to del Ponte, who joined in September, the diplomat said: "She brings a harder-edged prosecutorial lens so when they are looking at the evidence she is very well placed to know what sort of evidence would assist a later judicial process."
GENEVA (Reuters) -- Syrians in "leadership positions" who may be responsible for war crimes have been identified, along with units accused of perpetrating them, United Nations investigators said on Monday.