Only a few residents of Syria's Aleppo were able to leave encircled opposition-held districts through humanitarian corridors before rebels prevented them from fleeing, a monitor said Friday.

Russia, a key ally of President Bashar al-Assad, on Thursday announced the opening of aid passages for civilians and surrendering fighters seeking to exit the city's rebel-held eastern neighbourhoods.

Regime aircraft bombed eastern areas of Aleppo overnight, the Syrian Observatory for Human Rights said, without specifying casualties.

Entrances to the corridors were effectively shut in rebel areas inside the city on Friday, the Observatory said.

The other end of the passages, in regime-held territory, were open however, according to the monitor, which relies of a wide network of sources inside Syria for its information.

Since they were established "around 12 people managed to use the Bustan al-Qasr corridor before rebel groups reinforced security measures and prevented families from approaching the corridors," Observatory director Rami Abdel Rahman said.

Syria's opposition High Negotiations Committee on Thursday criticised the corridors, saying Russia and the regime aimed to "alter Aleppo's demographics and ensure forced displacement".

Pro-regime forces have surrounded Aleppo's eastern districts since July 17, sparking fears for an estimated 250,000 people who live there.

Rebel-held neighbourhoods have been effectively besieged -- with food shortages and price hikes -- since pro-regime forces completely cut off the opposition's main supply road into the city.

Analysts say that losing Aleppo would be a major blow for the armed opposition and could signal a turning point in the conflict, which began in 2011 with the brutal crackdown of anti-government protests.

Syria's five-year war has killed more than 280,000 people and displaced millions.