About 100 people have been killed in fighting in Syria, activists say, most of them in the two main cities, Damascus and Aleppo.
Several Aleppo neighbourhoods have been left without drinking water after a water main was damaged in fighting.
Separately, a rocket fired from western Syria killed a small girl over the border in Iraq, officials said.
It is one of the worst incidents across the Iraqi border since the Syrian conflict began in March 2011.
Meanwhile, the new UN-Arab League envoy Lakhdar Brahimi is set to begin mediation efforts, with a visit to Cairo for talks with Arab League foreign ministers.
In Aleppo, government forces are reported to have regained control of a barracks on the north-east side of the city, after a lengthy battle with rebels.
The rebels had seized control of the Hanano barracks, one of the largest army posts in the area, on Friday.
During the fighting, a major pipeline supplying water to Aleppo was badly damaged.
The city - the largest in Syria, with a population of about three million - is now suffering from shortages of drinking water, according to the Syrian Observatory for Human Rights, a UK-based opposition activist group.
"We cannot say how it was ruptured, but there were air strikes and clashes between the army and rebels in the area," Observatory director Rami Abdel Rahman told AFP news agency.
The BBC's Jim Muir in neighbouring Lebanon says video posted by activists from the Shaar quarter of Aleppo shows an apocalyptic scene of destruction in a street devastated by what they said were government air strikes.
The activists said several rebel-held parts of Aleppo were similarly bombarded.
In Damascus, government forces have shelled a southern district populated mainly by Palestinians.
Syria's Palestinian population stayed on the sidelines during the early months of the conflict, but recently Palestinian youths have been joining the rebels fighting the government of President Bashar al-Assad.
As the fighting continues, international envoy Lakhdar Brahimi is preparing for his first visit to the region since he took up his post last month.
He will hold several days of talks in Cairo before heading to Damascus.
On Saturday, EU foreign ministers meeting in Cyprus agreed to tighten sanctions on Syria, with diplomats saying new measures could be introduced next month.
However, our correspondent says diplomatic efforts to halt the carnage currently have no traction at all.
Activists say at least 23,000 people have been killed in the 18 months since the start of the conflict.