In northern Syria, rebels said they had begun a new assault on regime-held neighbourhoods of the country's second city, Aleppo.
"The district of Qaboon in (northeastern) Damascus is being intensely shelled by regime forces," the Syrian Observatory for Human Rights reported.
Rebels were resisting the onslaught, battling troops loyal to President Bashar al-Assad on the edges of Qaboon as the army tried to storm the neighbourhood, it said.
Overnight mortar fire targeting Qaboon killed three children from one family, the Britain-based group said.
The fighting erupted as rebels fought off an army attempt to enter the district, said the Syrian Revolution General Commission, a network of activists on the ground.
Battles also raged on the edges of Al-Hajar al-Aswad in southern Damascus and Jubar in the east, said the Observatory. Troops meanwhile pounded the Qadam district, parts of which are rebel-held.
Over the past fortnight, the army has dramatically boosted efforts to crush the anti-Assad insurgency in and around Damascus.
Meanwhile in Aleppo, large swathes of which are under rebel control, the main rebel Free Syrian Army (FSA) command announced the launch of "a battle to liberate several western districts".
Activists said fierce fighting raged in the regime-held New Aleppo district in the west of the city.
Rebels launched their first major assault on Aleppo on July 20 last year.
Ever since, despite many clashes and army bids to retake control, the city has fallen into a stalemate.
The latest assault is being staged by 13 rebel groups, including the powerful Liwa al-Tawhid and the Farouq Brigades, the FSA statement said.