GENEVA (AP) — At least eight massacres have been perpetrated in Syria by President Bashar Assad's regime and supporters and one by rebels over the past year and a half, a U.N. commission said Wednesday.

The commission's probe highlights the worsening pattern of violence against civilians, including executions and hospital bombings, as the government battles to retake lost territory from the rebels, including Islamist foreign fighters who also have carried out war crimes.

"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," says the report by the U.N. commission investigating human rights abuses in Syria.

The report updates the commission's work since 2011 to mid-July, stopping short of what the United States says was an Aug. 21 chemical weapons attack on rebel-held areas that killed hundreds of civilians.

Calling Syria a battlefield where armed forces are getting away with large-scale murder, the commission said that in each of the incidents since April 2012 "the intentional mass killing and identity of the perpetrator were confirmed to the commission's evidentiary standards."

The commission's four independent experts also note that they are probing nine more suspected mass killings since March. In those, it said, the illegal killing has been confirmed but the perpetrator cannot yet be identified. In other cases, it said, the circumstances of the killing were not sufficiently clear to be able to determine the legality.

"Relentless shelling has killed thousands of civilians and displaced the populations of entire towns. Massacres and other unlawful killings are perpetrated with impunity," the commission concludes. "An untold number of men, children and women have disappeared. Many are killed in detention; survivors live with physical and mental scars of torture. Hospitals and schools have been bombarded."

A confidential list of suspected criminals is being produced by the commission and kept under lock and key by U.N. High Commissioner for Human Rights Navi Pillay.

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"It's a long list," one of the commission's members, Carla del Ponte, told reporters on Monday.

The allegations of chemical weapons use received have been "predominantly by government forces," said the commission, but it has not yet been able to determine whether they were used. The commission is expected to eventually weigh in on who bears responsibility, after a separate team of U.N. chemical inspectors reports on any evidence of the use of chemical weapons. That report will not apportion blame.

Since the commission has not yet been given permission to carry out its work in Syria, the information in the latest report was based on interviews conducted in the region and from Geneva, including via Skype and telephone.

The commission said its team of about 20 investigators has carried out 2,091 interviews since September 2011. Most of the information is coming from the 2 million Syrian refugees streaming across the borders into neighboring countries.

The commission created by the U.N.'s 47-nation Human Rights Council says both sides have committed heinous war crimes during the 2 ½-year conflict that has killed over 100,000 people.

In a statement accompanying the report, the commission chaired by Brazilian diplomat and scholar Paulo Sergio Pinheiro said most casualties result from unlawful attacks using conventional weapons and any response to end the conflict "must be founded upon the protection of civilians."