Russian jets resumed heavy bombing of rebel-held eastern Aleppo on Tuesday after several days of relative calm, a rebel official and the Britain-based Syrian Observatory for Human Rights monitoring group said.
Air strikes mostly hit the Bustan al-Qasr neighborhood, Zakaria Malhifji of the Aleppo-based Fastaqim rebel group told Reuters.
"There is renewed bombardment and it is heavy," he said.
Rebel officials and the Observatory reported casualties from the air strikes but gave no immediate toll.
Moscow and Damascus reduced air raids in the northern city last week. The Syrian army said it was partly to allow civilians to leave opposition-held eastern neighborhoods.
The Syrian government said rebels holed up in Aleppo can leave with their families if they lay down their arms.
Insurgents denounced that offer as a deception.
President Bashar al-Assad seeks the complete recapture of Aleppo, Syria's biggest city before the 5-1/2-year war, and which has been divided between government and opposition control for years.
Assad's ally Russia has meanwhile built up its forces in Syria since a brief ceasefire collapsed last month.