Syrians in "leadership positions" who may be responsible for war crimes have been identified, along with units accused of perpetrating them, UN investigators say.
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 on Monday.
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 (ICC) for prosecution.
"The international community, and the UN Security Council, must take the decision to refer this to justice," Carla del Ponte, a former UN prosecutor and a member of the commission, said.
"We suggest the International Criminal Court."
War crimes on both sides
The list of suspects, building on lists drawn up in the past year, will remain a secret; it will be entrusted to Navi Pillay, the UN high commissioner for human rights.
Pillay, a former judge at the ICC, said on Saturday that Syrian president Bashar al-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," a European diplomat said.
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The death toll in Syria is by some estimates approaching 70,000 people, Pillay told the Security Council last week in a fresh appeal for it to refer Syria to the ICC.
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.
"Government forces and affiliated militias have committed extra-judicial executions, breaching international human rights law. This conduct also constitutes the war crime of murder."
They have also targeted queues at bakeries and funeral processions, in violence aimed at "spreading terror among the civilian population", and used cluster bombs, the commission said.
Rebel forces fighting to topple Assad have committed their own war crimes including murder, torture, hostage-taking and using children under age 15 in hostilities.
"They continue to endanger the civilian population by positioning military objectives inside civilian areas," the report said, adding that rebel snipers have 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," the report noted.
The UN team's report was unveiled as Syrian rebels seized a key army checkpoint near Aleppo airport amid continued fighting for control of several airports in the country's north, the UK-based watchdog Syrian Observatory for Human Rights (SOHR) reported.
"The rebels seized a checkpoint several hundred metres away from Nayrab military airport," on the road leading to Aleppo international airport, Rami Abdel Rahman of the SOHR told AFP news agency.
Rebels also clashed with troops along the airport road, he said.
The advance is the latest in a series by the rebels since they captured last week air bases at Al-Jarrah, Hassel and Base 80, also in Aleppo, a strategic province.
On February 12, rebels launched a battle to capture the main air base and international airport in Aleppo province.
The rebels are trying to capture the airport, which has been closed since January 1.
The Syrian army, meanwhile, is preparing to "cleanse" in the next 48 hours rebel-held areas in Aleppo, the pro-government Al-Watan newspaper reported.
Troops were now focused on "putting a stop to battles taking place around Base 80" which is tasked with securing the international airport and nearby Nayrab military base, Al-Watan said.
"The army should impose its control on Base 80 [after] cleansing areas seized by armed men," it said, adding that "the security situation in Aleppo is improving."