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.
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.
"We are now overlooking the Ramousah area but Russian jets are intensifying their bombing, which is holding us back from moving quickly," a rebel commander told Reuters news agency.
Another rebel source told Reuters that about 10,000 troops, at least 95 tanks and several hundred rocket launchers had been deployed for what he described as the "great epic battle of Aleppo".
The source said scores of suicide bombers had also been prepared to drive explosive-laden military vehicles into army posts.
UK-based monitor the Syrian Observatory for Human Rights described the rebel offensive as the largest so far, involving fighting across the divided city's main battle fronts.
It said government forces had regained two villages and two hilltops in the south-west suburbs of Aleppo late on Tuesday.
"The regime is launching counter-attacks to absorb the fierce rebel offensive," observatory head Rami Abdel Rahman said.
"Every day we have at least 20 to 30 attacks by Russian aircraft or by the helicopters.
"There are no cars going around. Public transport has completely stopped because of the lack of fuel. People are staying inside their houses to avoid shelling."
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.
Aleppo was once Syria's commercial capital and also boasted a rich architectural and archaeological heritage.
Much of it has been destroyed or looted during more than five years of war.
Russia and Syria have announced the opening of what they called humanitarian corridors for civilians and rebels wanting to surrender but few people are reported to have used them.