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."
A major winter storm has meanwhile brought severe weather to the war-ravaged country, delaying a UN aid airlift and compounding the misery of Syrians holed up in besieged towns and refugees sheltering in tents across the border in Lebanon.
The UN report released late Thursday cites "credible evidence" and "evidence consistent with the probable use of chemical weapons" at Ghouta, Khan al-Asal, Jubar, Saraqeb and Ashrafieh Sahnaya.
The mission "concludes that chemical weapons have been used in the ongoing conflict," said the report, prepared by a team led by Swedish expert Ake Sellstrom.
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 said there was clear evidence sarin gas was used in an attack east of the capital on August 21 that killed hundreds of people and sparked world outrage.
The United States threatened to strike Syria after the attack, but backed off after reaching a deal with Russia to dismantle Damascus's vast chemical arsenal.
Under the deal, enshrined in a Security Council resolution, Syria's most dangerous chemical weapons have to be out of the country by December 31 and destroyed by June 30, 2014.
The chemical arms deal is widely seen as having strengthened Assad's hand, and his forces have gathered momentum with a string of battlefield victories in recent weeks as Western-backed rebels have lost ground both to his forces and their Islamist rivals.
Over the past week, a powerful rebel alliance, the Islamic Front, seized the Bab al-Hawa crossing on the Turkish border and nearby weapons warehouses from the mainstream Free Syrian Army, prompting the United States and Britain to suspend aid to the group.
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, calling the recent setbacks a "big problem."
"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.
A former US Central Intelligence Agency chief meanwhile said a victory for Assad could be the best outcome to avoid a regional conflagration.
Michael Hayden, who headed the CIA until 2009, saw three possible outcomes -- a continuation of the civil war pitting ever more extreme Sunni and Shiite factions against one another, the "dissolution" of Syria or a victory by Assad.
"As ugly as it sounds, I'm kind of trending toward option three as the best out of three very, very ugly possible outcomes," Hayden told a conference of terror experts.
'Extreme cold' adds to Syrian misery
Assad loyalists are currently battling east of Damascus in a bid to expel rebels from the religiously mixed town of Adra, according to state news agency SANA.
The industrial town, the scene of fierce fighting for three days, is strategically located on a main road into the capital.
The fighting comes as Syrians already trapped in a war zone have had to contend with freezing temperatures, snow and rain as a severe winter storm has barrelled across the region.
A child and a baby were said to have died from the cold Thursday, and an activist in a besieged rebel-held town south of Damascus said residents were struggling to stay warm with the electricity cut off and no food or fuel allowed in.
"Normally we face the shelling and fighting, as well as food and fuel shortages. Today we also have snow and extreme cold," Abu Anas said by Internet from the town of Hara.
The severe weather forced the delay of a UN airlift from northern Iraq into Syria for a second consecutive day, though officials said conditions appeared to be improving.
Human rights group Amnesty International meanwhile said 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.