Syrian regime forces and rebel factions sent hundreds of reinforcements to Aleppo Monday as both sides braced for a crucial battle to control the country's second city.
Fighting for Syria's former economic powerhouse is intensifying after an opposition advance at the weekend broke through a three-week government siege of the city's rebel-held east, dealing a major setback to regime troops.
Rebel forces Sunday announced a bid to capture all of Aleppo city, which if successful would mark the biggest opposition victory yet in Syria's five-year civil war.
But forces loyal to President Bashar al-Assad are putting up a fierce fight and have begun pouring in reinforcements.
The main opposition coalition said Monday it was only a matter of time before rebels take all of Aleppo, but the United States warned there would be no quick victory.
The Syrian Observatory for Human Rights said some 2,000 pro-regime fighters from Syria, Iraq, Iran and Lebanese Shiite movement Hezbollah had arrived in Aleppo since late Sunday.
"Both sides are amassing their fighters in preparation for the great battle of Aleppo," Observatory head Rami Abdel Rahman said.
Al-Watan, a Syrian daily close to the government, reported Monday the army had received "the necessary military reinforcements to launch the battle to retake the areas from which it withdrew."
It said a military operation by Syria's armed forces was "imminent... and inevitable."
Aleppo has been roughly divided between government forces in the west and rebel groups in the east since fighting first broke out there in mid-2012.
- New phase -
An AFP correspondent in the eastern districts said shelling and sporadic clashes were hitting Aleppo but there were no signs yet of significant new offensives.
After years of stalemate, fighting for the city entered a new phase last month when government forces took control of the last supply road into rebel areas, surrounding some 250,000 people in eastern districts.
In a desperate bid to break the siege, a coalition of rebels, Islamists and jihadists overran a series of buildings in a military academy on the southwestern edges of Aleppo Saturday before linking up with rebel groups inside the city.
Emboldened by the victory, the fighters -- largely grouped under the banner of the Army of Conquest -- set their sights on recapturing all of Aleppo.
Signup to our newsletter and follow us on Facebook and Twitter!
The Army of Conquest Sunday announced "the start of a new phase to liberate all of Aleppo", pledging to "double the number of fighters for this next battle".
In Istanbul, Syrian National Coalition chief Anas al-Abdeh said he was confident the whole city could now be taken: "I think it is just a matter of time."
Washington's UN envoy Samantha Power said: "Neither side will be able to win a swift or decisive victory in the battle for Aleppo."
Abdel Rahman said hundreds of opposition fighters had arrived in Aleppo from the surrounding province and neighbouring Idlib.
Most were from the Fateh al-Sham Front, formerly Al-Qaeda affiliate Al-Nusra Front, that leads the Army of Conquest.
- Food and fuel -
"Whoever wins (in Aleppo), the war will not end. It is however an important battle, the result of which will set the course of the conflict," said Syria expert Thomas Pierret.
Pierret said a rebel win would confine the regime to an arc of territory between the western coastal areas and the Golan Heights, while a regime victory could lead to the "collapse" of the insurgency.
The rebel advance at the weekend cut off a key regime access route on the city's southern edges, previously used to ferry supplies to around 1.2 million residents in western districts.
Overnight, regime forces brought in trucks carrying food and fuel into western neighbourhoods via the northern Castello Road instead, the Observatory said.
More than 290,000 people -- including over 84,000 civilians -- have been killed since Syria's conflict erupted in March 2011, the Observatory said in a new toll Monday.
Doctors Without Borders (MSF) said that air strikes on a hospital in Idlib province on Saturday killed 13 people, including four staff and five children.
The hospital, which had served 70,000 people, was destroyed, the medical charity said Monday.
International efforts to resolve the conflict have repeatedly failed, although the UN is hoping that peace talks can resume later this month.
The efforts come as a US-led coalition fights the Islamic State jihadist group, which seized large swathes of Syria and neighbouring Iraq in 2014.
A coalition air strike Sunday destroyed 83 oil tankers used by IS in the eastern province of Deir Ezzor, the US military said.