Elite government forces backed by Hezbollah converged Tuesday on the north Syrian city of Aleppo as rebels bolstered their own fighters in readiness for a major showdown, a monitor said.
Rami Abdel Rahman of the Syrian Observatory for Human Rights said it was likely that pro-regime forces were preparing to lay siege to rebel-held parts of Aleppo, the country's pre-war commercial capital.
The rebels fighting to oust President Bashar al-Assad have controlled parts of the northern city since they wrested them from the government's hands in July 2012.
"Commando troop reinforcements from the Republican Guard and from (Lebanon's) Hezbollah continued to be sent, in a bid to surround Aleppo city," the Observatory said.
At the same time, "rebel and (allied) Islamist brigades sent reinforcements of their own... through the eastern entrance to Aleppo city," added the Britain-based organisation.
Aleppo was Syria's most populous city before the conflict, but it is now a major battle zone split into areas controlled by the rebels concentrated in the east and those held by the government mainly in the west.
The rebel-held areas have come under daily aerial bombardment since December.
The Observatory says the offensive has killed some 2,000 civilians, including more than 500 children, since January.
It has also forced tens of thousands of families to flee.
- Air strikes kill child -
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On Tuesday, fresh air strikes hit Marea, a rebel town in Aleppo province, as well as Tal Rifaat village nearby, killing a child, said the Observatory.
A mortar struck the government-held district of Ashrafiyeh, in Aleppo city, it added.
The developments come two days after the opposition National Coalition warned the regime is preparing to launch a major assault on Aleppo.
Compounding the rebels' fears are recent advances throughout the north and east of Syria by the radical Islamic State (IS), which recently declared the establishment of a "caliphate" straddling Syria and Iraq.
Rebels have been fighting IS since January.
The jihadist group was initially welcomed in Syria by some opponents as potential allies, but its systematic abuses and quest for domination has turned the rebels -- even the Islamists -- against them.
IS has recently been emboldened by a lightening offensive it has spearheaded in Iraq, where it has captured vast amounts of heavy weapons from fleeing Iraqi troops.
Rebels have frequently said IS's interests overlap with the regime's.
On Monday, Abdul Ilah al-Bashir, who heads the Free Syrian Army's Supreme Military Council, said the regime and IS "complete each other".
His statement came a day before a convoy of 50 vehicles that belong to a group in the northwestern province of Idlib that has pledged loyalty to IS travelled through regime-held areas to jihadist-held Raqa in the north and Al-Bab near Aleppo, according to the Observatory.
"Just as the regime is advancing on Aleppo city, IS is advancing on its countryside. The regime is happy the rebels have to fight on two fronts," said Abdel Rahman.
Syria's war began as a peaceful movement calling for political change, but morphed into a war after Assad's regime unleashed a massive crackdown on dissent.