Syria vowed Friday to crush rebels on an eastern approach to Damascus after a string of regime advances that have galvanised Islamist fighters and largely sidelined the Western-backed opposition.
The advance on the town of Adra came as a new UN report concluded that chemical weapons had been used at least five times before Damascus agreed to give up its arsenal, and as Washington expressed concern about recent rebel setbacks.
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.
In Adra, an industrial town located on the main road into the east of the capital, fighting raged for a third consecutive day as the army warned in a televised statement that "the operation will continue until the terrorists are crushed completely."
Friday's fighting comes two days after Islamist rebels have closed ranks to attack security, army and pro-regime militia positions in the town.
At least 18 pro-regime fighters and 15 civilians were killed between Wednesday and Thursday in Adra.
The Islamist-led offensive highlighted the increasingly marginal role played by the Western-backed Free Syrian Army.
Over the past week, a powerful new rebel alliance, the Islamic Front, seized a Turkish border crossing and nearby weapons warehouses from the mainstream FSA, prompting the United States and Britain to suspend aid to the group.
US Defence Secretary Chuck Hagel said Washington would continue to support "moderate" forces but will withhold non-lethal aid until it can assess who controls arms depots and border crossings, calling the recent setbacks a "big problem."
In Aleppo province, meanwhile, the Al-Qaeda-affiliated Islamic State of Iraq and the Levant (ISIL) kidnapped 120 Syrian Kurds, including six women, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.
ISIL is already holding scores of Kurds hostage, as well as dozens of other Syrian activists and some two dozen foreign journalists.
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In Geneva, UN High Commissioner for Human Rights Navi Pillay described the rise in kidnappings as a reflection of "a callous indifference to human life" and called on all parties to "stop terrorising civilians through abduction."
Chemicals used at least 5 times
The UN report released late Thursday cites "credible evidence" and "evidence consistent with the probable use of chemical weapons" in five locations in and around the capital, as well as in Aleppo and Idlib in the north.
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 Syrian President Bashar al-Assad accuse his regime of carrying out the attacks. Assad, 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 UN 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, including in the strategic Qalamoun region near the Lebanon border.
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, delaying a planned UN aid flight from Iraq for a second consecutive day.
A Syrian child and a baby were said to have died from the cold Thursday, and an activist in a besieged rebel-held area of a town south of Damascus said residents were struggling to stay warm with 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.