Barrels bombs hit Syria's Aleppo on Thursday just hours after a temporary truce announced by regime ally Russia came into effect in the northern war-torn city.
An AFP reporter said regime helicopters dropped the crude explosive devices on the city's rebel areas after residents had headed to markets for their first morning shopping in weeks.
Aleppo has seen some of the worst fighting in a war that has killed more than 280,000 people, and there is deep skepticism that the latest halt to fighting in the battered city will last.
The two-day truce came hours after U.S. Secretary of State John Kerry warned Moscow that Washington's patience was running out over breaches of a nationwide ceasefire.
Peace talks aimed at ending the five-year conflict have stalled and a February countrywide ceasefire between the regime and non-jihadist rebels lies in tatters.
"There is no progress in the political process," Russian Foreign Minister Sergei Lavrov said at an economic forum in Saint Petersburg, referring to Syria.
He accused Washington, which supports Syrian rebels, of being "unable or unwilling to put pressure on its allies in the region."
Nevertheless, direct contact between Russia and the United States about Syria have taken place "without any hysteria," he added.
There have been repeated violations of the February 27 truce in Aleppo, with rebels pounding regime-controlled neighborhoods with rocket and artillery fire and the regime hitting rebel areas with air strikes.