Intense fighting has continued around the Syrian city of Aleppo, where a rebel offensive is trying to break a government siege of rebel-held areas, BBC News reports.

Over the weekend, the rebels tried to reconnect an encircled area in the east with insurgent territory in the west.

They set off a huge tunnel bomb underneath army positions in the strategic Ramouseh district.

The army has been fighting back with the help of Russian air strikes to stop the rebels breaking through.

Around a quarter of a million civilians are living under siege in rebel-held areas since government forces cut them off last month.

The UK-based Syrian Observatory for Human Rights has described the rebel offensive as the largest so far, involving fighting across the divided city's main battle fronts.

Government troops supported by Russian warplanes have put up a fierce defence, the observatory adds.

The observatory said 50 rebel fighters and jihadists had been killed since the operation began on Sunday, as well as dozens of government soldiers.

It has been reported that barrels suspected to contain chlorine gas were dropped on the town of Saraqeb near Aleppo.

About 30 people, mostly women and children, were reported to have been affected. Chlorine gas can cause victims to become short of breath and to foam blood from the mouth.

It is not clear who was responsible.

The attack took place near to where a Russian military transport helicopter was shot down by rebels hours earlier killing all five personnel on board.

It was the deadliest single incident for Russia's military since its air campaign began last September.

The Russian government has responded to US criticism over its actions in Syria by arguing that it is "unacceptable" to demand restraint around Aleppo.