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.