Barrel bombs are explosive-filled cylinders or oil barrels, often rolled out of the back of helicopters with little attempt at striking a particular target but capable of causing widespread casualties and significant damage.

The Britain-based Observatory said that rebel groups in Aleppo issued a statement asking civilians in government-held parts of the city to move away from state security buildings, which they said would be targeted in retaliation for the bombings.

President Bashar al-Assad's forces, battling rebels in a 2-1/2 year conflict that has killed more than 100,000 people, frequently deploy air power and artillery against rebel-held districts across the country.

They have been unable to recapture eastern and central parts of Aleppo, which rebels stormed in the summer of 2012, but they have driven rebel fighters back from towns to the southeast of the city in recent weeks.

The conflict has grown sectarian, with majority Sunni rebels battling Assad's own Alawite sect and Shi'ite militia.

The Observatory - which has a network of opposition, pro-government and medical sources - also said on Monday that rebels in northern Aleppo province were threatening to strike two Shi'ite villages they have surrounded with missiles if barrel bombs were used again by the army.

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