BEIRUT (Reuters) - The Damascus water authority has been forced to cut supplies coming into the Syrian capital for a few days and use reserves instead after rebels polluted the water with diesel, it said on Friday.

The al-Fija spring which supplies Damascus with water is in the rebel-held Wadi Barada valley northwest of the capital in a mountainous area near the Lebanese border.

The government controls much of the surrounding territory and on Friday carried out aerial attacks and shelled the rebel-held area, the Syrian Observatory for Human Rights said.

A military news outlet run by Syrian government ally Hezbollah said the rebels in the Wadi Barada valley had refused to leave the area and as a result the Syrian Arab Army began an offensive against them on Friday morning.

Through a series of so-called settlement agreements and army offensives, the Syrian government, backed by Russian air power and Iran-backed militias, has been steadily suppressing armed opposition around the capital.