Regime and rebel forces have sent in hundreds of fighters and extra military equipment to join the crucial battle for Syria's second city Aleppo, a monitoring group said Monday.

Fighting is intensifying as both sides prepare for what Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, said would be a decisive battle for the northern city.

"An estimated 2,000 Syrian, Iraqi, and Iranian fighters as well as Lebanese fighters from (Shiite movement) Hezbollah have arrived in Aleppo since Sunday night," Abdel Rahman told AFP.

The Monday edition of Al-Watan, a Syrian daily close to the government, said the army and allied forces had received "the necessary military reinforcements to launch the battle to retake the areas from which it withdrew".

Al-Watan said a Palestinian loyalist militia had sent "huge reinforcements" to Syria's army battling to defend a cement factory south of Aleppo.

Citing a source on the ground, the newspaper said military warplanes "are carrying out a barrage of air strikes targeting the armed groups."

Forces loyal to President Bashar al-Assad are on the defensive in Aleppo since an opposition alliance overran key territory south of the city at the weekend.

The coalition of rebels, Islamists, and jihadists seized territory in a military academy on Aleppo's edges on Saturday, breaking the government encirclement of eastern districts, home to an estimated 250,000 people.

"Hundreds of fighters, specifically from the Fateh al-Sham Front (formerly Al-Qaeda's Syrian branch), and others arrived in Aleppo city from other parts of the province and Idlib province," said Abdel Rahman.

Fateh al-Sham Front -- which changed its name from Al-Nusra Front after breaking off ties with Al-Qaeda -- leads the Army of Conquest, which has played a major role in the fight for Aleppo.

Late Sunday, the Army of Conquest pledged to "double the number of fighters" to recapture all of Aleppo city.

"Both sides are amassing their fighters in preparation for the great battle of Aleppo... which is existential for both the fighters and their backers," said Abdel Rahman.

Once Syria's commercial hub, Aleppo has been transformed into a bombed-out, divided city since fighting first erupted there in 2012.