Residents of Aleppo, Syria's largest city, have been without water for a week because fighters have cut supplies into rebel and government-held areas, a monitoring group has said.

The Syrian Observatory for Human Rights said on Sunday that the al-Qaeda affiliate al-Nusra Front had cut water supplies from a pump distributing to both the rebel-held east and government-held west of Aleppo.

But anti-government fighters blame the Syrian air force for bombing water supply lines.

Last month, opposition forces cut the electricity supply to government-controlled areas of Aleppo and the surrounding countryside.

But Observatory director Rami Abdel Rahman said the groups were unable to cut off water supplies to government areas without also affecting rebel-held neighbourhoods, calling the move "a crime".

The Observatory said the week of water cuts had forced residents to queue in front of wells to collect water, and the Britain-based group warned that some people were drinking unclean water risking a spread of disease.

Once home to almost 2.5 million residents and considered Syria's economic powerhouse, Aleppo has been divided between government and opposition control since shortly after fighting there began in mid-2012.

At least one million people have been displaced from the city since then by fighting and relentless government aerial bombardments of rebel areas.

Opposition forces also regularly shell government-held parts of the city in the west.