Syrian president Bashar al-Assad's air force has been pounding rebel-held areas of Aleppo with barrel bombs, according to observers. Helicopters drop crude bombs — barrels packed with explosives, fuel and scraps of metal — on the city, causing immense damage and indiscriminate loss of life.

Over a two-week stretch in December alone, activists say airstrikes killed more than 500 people.

Syria's opposition has pointed to the air raids as evidence that Assad has little interest in peace despite sending a delegation to Switzerland for U.N.-sponsored negotiations.

Since it began in March 2011, the Syrian conflict has killed more than 130,000 people and forced almost a third of the country's population of 23 million from their homes.

The use of barrel bombs across Syria has been widely condemned by human rights groups because of the weapons' indiscriminate nature.

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February 4, 2014: A Civil Defence officer extinguishes a fire at a power station caused by what activists said were explosive barrels thrown from helicopters by forces loyal to Syria's President Bashar Al-Assad, in Al-Hawoz neighbourhood of AleppoReuters
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February 4, 2014: A man pulls a trolley containing belongings at the Karaj al-Hajez crossing, a passageway separating Aleppo's Bustan al-Qasr, which is under the rebels' control, and Al-Masharqa, an area controlled by the regime. Reuters photographs show streets packed with residents fleeing the home-made barrel bombs - oil drums or cylinders packed with explosives and metal fragments. Barrel bombs, cheap and easy to construct and usually dropped from helicopters, often kill dozens at a time. Activists say they are being used to push people from rebel-held areas into state-controlled parts of the cityReuters
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February 3, 2014: A man holds an injured boy who survived what activists said were barrel bombs dropped by government forces, in Al-Shaar, AleppoReuters
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February 2, 2014: Residents point to the sky during as activists said were explosive barrels were dropped by forces loyal to Syria's President Bashar al-Assad in the Tariq Al Bab neighbourhood of AleppoReuters
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February 2, 2014: Residents and activists carry an injured girl out of the debris after what activists said was a barrel bomb attack in the Tariq Al Bab neighbourhood of AleppoReuters
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February 1, 2014: A Syrian Army helicopter drops what appears to be an explosive barrel, on the Al-Shaar area of AleppoReuters
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v: Smoke rises after a reported barrel bomb attack by government forces in Daraya, near DamascusReuters
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January 21, 2014: A man reacts near a site hit by what activists say were barrel bombs dropped by government forces, on al-Katerji district in AleppoReuters
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December 17, 2013: People look at damage caused by what activists said was an airstrike with explosive barrels in Al-Shaar, AleppoReuters
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December 23, 2013: A Free Syrian Army fighter gestures at a site hit by what activists said was an air raid by forces loyal to Syrian President Bashar Al-Assad, in Aleppo's al-Saliheen districtReuters
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December 23, 2013: A man reacts after what activists said was an explosive barrel air raid by forces loyal to Syrian President Bashar Al-Assad, in Aleppo's al-Marja districtReuters
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December 22, 2013: Residents inspect a blast crater after after reported barrel bomb attack at Masaken Hanano in AleppoReuters
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December 17, 2013: People look at buildings damaged in what activists said was a barrel bomb airstrikeReuters
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December 17, 2013: Free Syrian army members remove a body from rubble caused by what activists said was an airstrike with explosive barrels in the Al-Shaar area of AleppoReuters
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September 7, 2013: A barrel lies on the ground in the Ariha countryside. Activists said it was thrown from Syrian government aircraftReuters