control in the west since shortly after fighting began there in mid-2012.
In the surrounding countryside the situation is largely the reverse, with rebels controlling much of the area west of the city and regime forces much of the east.
Government forces advanced around the east of the city last year, but the front lines had been relatively static in recent weeks.
On Monday, the Observatory reported an influx of regime reinforcement and the Syrian daily Al-Watan, which is close to the government, said regime forces planned to encircle the city in a new offensive.
“Aleppo is very important for us,” a Syrian military source told AFP on Tuesday.
“The main goals are to break the siege of Aleppo and open the road to Nubol and Zahraa,” he added.
The new offensive comes shortly after regime forces opened a new front in southern Daraa province. “This military operation in Aleppo proves the ability of the Syrian army to open multiple fronts at once,” the military source said.
Noah Bonsey, a Syria expert at the International Crisis Group, said the offensive represented a potentially serious escalation by the regime.
“If the regime were able to take these villages, and if it can hold them and break the siege on Nubol and Zahraa, these would be very significant developments taken together,” he told AFP.
The offensive began as UN envoy de Mistura prepared to address the Security Council.
He has advanced a plan for a “freeze” to the fighting in Aleppo, in a bid to ease the humanitarian situation and provide an example for ceasefires elsewhere.
But the proposal has gained little traction, and de Mistura drew criticism from the opposition last week after describing Syria’s President Bashar al-Assad as “part of the solution” to the conflict.
The rebels and opposition insist Assad’s departure is a precondition for resolving the country’s brutal war, which began in March 2011 with peaceful anti-government protests.
It spiralled into a civil conflict after a government crackdown, and the violence has killed more than 210,000 people.