Syrian government air strikes and artillery fire killed at least 16 civilians on Wednesday in rebel-held neighbourhoods in the east of Aleppo city, a monitor said.

The Britain-based Syrian Observatory for Human Rights said seven people had been killed in the Sakhur neighbourhood and another seven in Al-Ansari district.

Two more were killed in the Sukari and Bustan al-Qasr neighbourhoods, the monitor said, adding that the toll could rise because people were still trapped under the rubble.

The renewed strikes came as Syria's army officially announced it had surrounded the rebel-held east of the city.

Opposition neighbourhoods of Aleppo have been effectively besieged since July 7, when government troops advanced to within firing range of the sole remaining supply route into the east.

They have continued to advance, seizing the road itself and at least one rebel-held neighbourhood in the north-west of the city.

"Our armed forces... cut all the supply routes and crossings used by terrorists to bring mercenaries, weapons and ammunition into eastern neighbourhoods of Aleppo," the army said in a statement carried by official media.

It added that "in order to stop the bloodshed" fighters were being offered the opportunity to lay down their arms, "settle their situations" and remain in or leave Aleppo.

A day earlier the military said it had sent text messages to residents and fighters in Aleppo urging rebels to lay down their weapons and identifying "safe passages" for civilians wishing to leave.

Once Syria's economic powerhouse, Aleppo has been roughly divided between rebel control in the east and government control in the west since mid-2012.

More than 280,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests that were met with a regime crackdown.