BEIRUT/UNITED NATIONS: 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 over 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 5-year-old civil war. But forces loyal to President Bashar 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 Britain-based Syrian Observatory for Human Rights said some 2,000 pro-regime fighters from Syria, Iraq, Iran and Lebanese 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.

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 300,000 people in eastern districts.

In a desperate bid to break the siege, a coalition of rebels, Islamists and militants 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. 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.”

Washington’s U.N. 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 neighboring Idlib.Most were from the Fatah al-Sham Front, formerly Al-Qaeda affiliate the Nusra Front, that leads the Army of Conquest. “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,” Syria expert Thomas Pierret said.

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 neighborhoods via the northern Castello Road instead, the Observatory said.

The United States warned the United Nations Security Council Monday it must not allow civilians on both sides of the Syrian city of Aleppo to be cut off from humanitarian aid.

The United States, Britain, France, New Zealand and Ukraine organized an informal Security Council meeting on Aleppo Monday with briefings by a “White Helmet” rescue worker and two U.S.-based doctors from the Syrian American Medical Society who recently returned from Aleppo.

“If the fighting continues it is conceivable that civilians on both sides of Aleppo could be cut off from the basic assistance they need. We cannot allow this to happen,” Power said.

Citing U.N. figures, Power said Syrian government forces were to blame for nearly 80 percent of the besieged areas throughout Syria.

Doctors Without Borders (MSF) said that airstrikes on a hospital in Idlib province 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.