BEIRUT -- Syrian government helicopters and warplanes unleashed a wave of airstrikes Sunday on more than a dozen opposition-held neighborhoods in the northern city of Aleppo, firing missiles and dropping crude barrel bombs in an attack that killed at least 36 people, including 17 children, activists said.
Aleppo has been a key battleground in Syria's civil war since rebels swept into the city in mid-2012 and wrested most of the eastern and southern neighborhoods from the government. Since then, the fighting has settled into a bloody grind, with neither side capable of mounting an offensive that would expel its opponents from the city.
But over the past two months, President Bashar Assad's air force has ramped up its aerial campaign on rebel-held areas of Aleppo, pounding them with barrel bombs -- containers packed with explosives, fuel and scraps of metal -- that cause massive damage on impact.
On Sunday alone, Syrian military aircraft targeted 15 opposition-controlled neighborhoods, said an activist who goes by the name of Abu al-Hassan Marea.
The Observatory put the day's death toll in the air raids at 36, including 17 children. Mr. Marea said more than 50 people were killed in the airstrikes, although he did not have an exact count.
This is not the first time that Assad's air force has waged an intense campaign over Aleppo. In December, military helicopters pounded rebel-held districts of the city with barrel bombs, leveling buildings, burying people under the rubble and killing more than 500 people over a two-week stretch.
The misery in Aleppo was then compounded in early January by an outburst of rebel-on-rebel fighting, which has weakened the opposition's grip on parts of the city.
The rebel clashes have killed more than 1,400 people since they began a month ago, and the fighting shows little sign of coming to an immediate close.
On Saturday, a twin suicide bombing killed 26 people, including a senior military commander of the Tawhid Brigade, a prominent rebel group opposed to the Islamic State.
The attack, widely blamed by both pro- and anti-al-Qaida activists on the Islamic State, targeted the base of its rivals in the Tawhid Brigade and killed senior leader Adnan Bakkour, said Observatory director Rami Abdurrahman.
The Islamic State also killed another prominent commander, Abu Hussein al-Dik of Suqour al-Sham, on Saturday near the central city of Hama, the Observatory said. Mr. Abdurrahman said Mr. Dik was killed in an ambush outside of Hama.