At least 30 people have died, including eight children, in the last 24 hours in Aleppo, a city seeing some of the worst of a renewed escalation between government and rebel forces in the Syrian war.
Intensified fighting has all but destroyed a partial ceasefire that started at the end of February, with UN-led peace talks in disarray.
In Aleppo, divided between areas controlled by the government and by rebels, 19 people were killed by rebel shelling and 11 were killed by government air strikes, the Syrian Observatory for Human Rights said.
That adds to another 60 people killed over the weekend in Aleppo, Syria's largest city before the war, according to the Observatory.
Air strikes were also reported in rebel-held areas near Damascus and in Hama province on Tuesday.
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In a separate incident 25km west of Aleppo, five Civil Defence workers - first responders in opposition-held territory where medical infrastructure has all but broken down - were killed by air strikes and a rocket attack on their centre.
It was unclear whether Syrian or Russian warplanes had launched the raids.
Each side accuses the other of targeting civilian areas in the five-year-old war that has killed more than 250,000 people.
A Syrian military source said the army would "respond firmly" against rebels attacking government-held parts of Aleppo.
State news agency SANA said terrorist groups, including the al Qaeda-affiliated Nusra Front, had shelled neighbourhoods.
In the north of Aleppo, insurgents resumed bombardment of a Kurdish-controlled neighbourhood, Sheikh Maqsoud, according to the Kurdish YPG militia.
The YPG and its allies have been battling rebels, including groups backed via Turkey by states opposed to President Bashar al-Assad, for several months near Aleppo and close to the Turkish border.
Rebels accuse the YPG of collaborating with the government in trying to stop people using the only road into opposition-held Aleppo, something the YPG denies.
