Syrian regime forces and militants of the allied Lebanese Shiite movement Hezbollah started a wide-scale attack on rebel-held areas north-west of Damascus on Tuesday, which include the capital's water supply, according to a monitoring group.
"The intensified attack started in the morning, but it picked up in the late evening hours, under the cover of heavy artillery and aerial shelling," Rami Abdel Rahman, the head of the British-based Syrian Observatory for Human Rights said.
He added that fighting was taking place around the Ein al-Fijehspring, the source of the Syrian capital Damascus' water supply.
Activists inside the region of Wadi Barada said the regime "is targeting Wadi Barada villages with its warplanes and heavy artillery."
The area had seen relative calm last week amid negotiations for the repair or handover of the spring and nearby water-pumpingstations, currently held by rebels.
The capital's water supply has been cut off since December 22 and the city is relying on dwindling reserves, with both sides trading blame for the shortage.
The surge in violence comes during a shaky Russian and Turkish-brokered ceasefire that went into effect in Syria on December 30. The nationwide truce is aimed at stopping Syria's conflict of nearly six years.