Forces loyal to President Bashar al-Assad's regime "have continued to conduct widespread attacks on the civilian population, committing murder, torture, rape and enforced disappearance as crimes against humanity," according to the report, which covers the period from May 15 to July 15.

It also charged that anti-government groups had "committed war crimes," including murder, torture and hostage-taking.

Since the beginning of the year, the commission concluded that government-loyal forces were responsible for eight massacres, while opposition forces had committed one. Nine others were still under investigation.

The report does not address the period after July 15, which includes the August 21 suspected chemical attack on civilians near Damascus that sparked international outrage and could still lead to US-led military strikes, despite intense diplomatic efforts to avoid them.

In Wednesday's statement though, the four-member commission led by Brazilian Paulo Sergio Pinheiro insisted on the "need for accountability, both to bring to justice those who used them (chemical weapons, if confirmed) ... and to deter anyone else from using these abhorrent methods of warfare."