The Syrian opposition said Saturday that their fighters have broken a Syria regime siege on opposition-held areas in the northern city of Aleppo.

Jaish al-Fatah (Army of Conquest) fighters seized control of regime positions in Ramousa area in southern Aleppo, opening the route to opposition-held neighborhoods in the city, opposition sources told Anadolu Agency, requesting anonymity due to security concerns.

According to the sources, groups of opposition fighters have arrived into besieged neighborhoods in Aleppo.

"Clashes are still raging on in Ramousa outskirts between regime forces and opposition groups, which are trying to widen their control," the sources said.

A quarter of a million civilians still live in Aleppo's opposition-controlled eastern neighborhoods, effectively under siege since the army, aided by Iranian-backed militias, cut off the last road into rebel districts in early July.

Fighters from a coalition of rebel groups called "Jaish al Fateh" that includes Jabhat Fateh al Sham, the former al Qaeda-affiliated Nusra Front, Ahrar al Sham and other smaller groups, said they had taken the main fortress-like artillery academy in the Ramousah quarter in southwestern Aleppo.

They were now fighting to take the other military academies adjoining the artillery base that are among the country's largest."The Army of Conquest on Saturday took control of the armament school, where there is a large amount of ammunitions, and a large part of the artillery school", the Syrian Observatory for Human Rights said.

The coalition "is about to cut off, by gunfire, the supply route into regime-controlled districts" of the city, Observatory head Rami Abdel Rahman said.

The road, which passes through a southwestern suburb of Aleppo city called Ramussa, is the last open route into Aleppo city.

If the rebels capture it, they would simultaneously cut off regime forces and open up a new road for their own besieged forces in the city's east.

The former Al-Nusra Front -- renamed Jabhat Fateh al-Sham after breaking from Al-Qaeda -- on Saturday announced having captured the two military academies and a third military position.

State television, meanwhile, reported fighting in the three locations.

"The regime forces are in a very difficult position despite Russian air support," Abdel Rahman said.

Once Syria's economic powerhouse, Aleppo has been roughly divided into a rebel-held east and regime-controlled west since 2012. The opposition advance comes three weeks after regime forces backed by Russian air strikes besieged the city's opposition-held districts, raising fears of a humanitarian crisis there.

The artillery base is almost 2 km from the besieged opposition area. It has a huge supply of ammunitions and is used regularly to shell parts of the city held by opposition forces.

The rebels are trying to break through a strip of regime-controlled territory to reconnect their encircled sector of eastern Aleppo with a swathe of insurgent territory in the west of Syria, effectively breaking the siege.

The fall of that strip would also cut off western Aleppo, which is in regime hands.

"There are two suicide bombers who have driven into regime posts inside the artillery base," said Abu al-Walid, a fighter with Ahrar al Sham, who said there was fighting inside the base.

Hundreds of fighters were clashing with regime troops only a few hundred meters from each other in parts of the artillery base after breaking into regime defenses around the heavily fortified compound, rebels said.

Aleppo, Syria's biggest city before the outbreak of the civil war five years ago, has been divided between regime forces and rebels since the summer of 2012.

Seizing full control would be the biggest victory for Bashar al-Assad in five years of fighting and demonstrate the dramatic shift of fortunes in his favor since Russia joined the war on his side last year.

Rebels have poured in thousands of fighters mainly from the rebel-held province of Idlib in north western Syria and deployed dozens of tanks and armored vehicles in the operation that was named the "Epic battle of Aleppo".

Inside the city, Free Syrian Army (FSA), among them vetted U.S.-backed groups, helped pile pressure on the army and its allies along other frontlines.

Foreign opponents of Assad including Saudi Arabia and Turkey have been supplying vetted rebel groups with weapons via a Turkey-based operations center.

Some of these groups have received military training overseen by the U.S. Central Intelligence Agency. The vetted groups have been a regular target of the Russian air strikes.

Jabhat Fateh al Sham, which is believed to have carried out at least three suicide bombings so far, said it also killed a number of Lebanese Shiite Hezbollah fighters it said were defending the artillery school.

The militant Shiiite group that fights alongside Assad's regime forces is an ally of Iranian-backed militias and the Russians in trying to help Assad regain control of the opposition-held parts of Aleppo.

The deputy head of the powerful Lebanese group, Sheikh Naim Qassem, said in an interview with Reuters this week he saw no immediate end to the war in Syria.

The army said it had foiled the attack on the artillery base and two major military academies. Hundreds of insurgents had been killed and much of their armored vehicles and tanks destroyed, the army said. It said the assault was the biggest by rebels against regime-held areas in the last few years.

"Today there was a large scale attack by the terrorist armed groups and they used all types of weapons but were are fighting this attack and will defeat them," said Brigadier General Deeb Bazi, the head of one of the military academies targeted.

The army said at least a thousand insurgents had been killed since the assault began earlier this week.