Syrian opposition groups launched an attack against regime positions in the historic centre of Aleppo on Monday in response to an offensive that cut a road leading into the opposition-held sector of the city.

The attacks, which involved the shelling of regime-held neighbourhoods and intense street fighting, came just days after the advance by the regime side towards the Castello Road.

Opposition were relying on the road for supplies and access, and its severing effectively put opposition-held areas under siege, where at least 250,000 people live.

The city, Syria's largest before the war, has been a main battleground in the country's five-year war, and has seen an escalation of fighting after peace talks and a fragile ceasefire broke down earlier this year.

Its capture would be a strategic prize for the Assad regime, which controls the major population centres in western Syria, apart from opposition-held areas of Aleppo, and the city of Idlib. Opposition also control pockets of territory elsewhere in western Syria. Early on Monday more than 300 shells fired by opposition groups hit western, regime-held neighbourhoods in Aleppo killing five people and wounding dozens more, observors said.

Elsewhere, regime forces also attempted to advance in Eastern Ghouta and achieve a breakthrough in the countryside of Damascus amid intensive shelling and airstrikes.The U.S. and Russia brokered a nationwide cease-fire in February, which held for a time but then collapsed under pressure of localized fighting and a failed political process to end the violence. There have been attempts since then to reach short-term cessations of hostilities that are often region specific, but they have tended to be short lived.