A UN report has concluded chemical weapons were used at least five times before Syria agreed to dismantle its arsenal, as Washington called setbacks for moderate rebels a "big problem".
The report cites "credible evidence" and "evidence consistent with the probable use of chemical weapons" at Ghouta, Khan al-Asal, Jubar, Saraqeb and Ashrafieh Sahnaya.
"The United Nations Mission concludes that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic," said the report, prepared by a team led by Swede Ake Sellstrom.
However, the report does not attribute blame for the attacks, as this was beyond the team's mandate from the UN Security Council.
Syrian President Bashar al-Assad has admitted his forces hold chemical weapons, and has vowed to surrender them to international experts.
But he also insists his forces did not target civilians.
Western and Arab governments, human rights groups and the rebels fighting to oust Assad accuse his regime of carrying out the attacks. Assad and his allies Russia and Iran blame the rebels.
The UN report concluded that banned chemical weapons had been widely used and there was clear evidence sarin gas was used in an attack in a district east of the capital known as Eastern Ghouta on 21 August.
It was that incident that sparked international outrage and threats of US military action against Assad's regime.
Under an international agreement brokered to avoid US military strikes -- which resulted in a landmark Security Council resolution -- Syria's most dangerous chemical weapons have to be out of the country by 31 December and destroyed by 30 June, 2014.
The report has been distributed to Security Council members, and UN chief Ban Ki-moon is due to present it to the General Assembly later on Friday.
The United States, meanwhile, has called military setbacks for Syria's Western-backed opposition a "big problem".
Washington would continue to support "moderate" forces but will withhold non-lethal aid until it can assess who controls arms depots and border crossings, Pentagon chief Chuck Hagel told reporters.
"I think what has occurred here in the last couple of days is a clear reflection on how complicated and dangerous this situation is and how unpredictable it is," Hagel said.
Over the past week, a powerful rebel faction, the Islamic Front, seized the Bab al-Hawa crossing on the Turkish border and nearby weapons warehouses from the mainstream Free Syrian Army.
Meanwhile, a former US Central Intelligence Agency chief said a victory for Assad could be the best outcome to avoid a regional spillover.
Michael Hayden, who headed the CIA until 2009, saw three possible outcomes.
The first was for conflict between ever more extreme Sunni and Shiite factions, and the second, which he deemed the most likely, was the "dissolution of Syria".
"Option three is Assad wins," Hayden told the annual Jamestown Foundation conference of terror experts.
"And I must tell you at the moment, as ugly as it sounds, I'm kind of trending toward option three as the best out of three very, very ugly possible outcomes," he said.
Human rights group Amnesty International said on Friday European leaders should "hang their heads with shame" over their treatment of Syrian refugees.
An Amnesty briefing said EU member states have offered only around 12,000 places to Syrians as part of the UN High Commissioner for Refugees' goal of 30,000 places.
"The EU has miserably failed to play its part in providing a safe haven to the refugees who have lost all but their lives," Amnesty chief Salil Shetty said.
"The number of those it's prepared to resettle is truly pitiful," he added.