Their desire to force a shift in the balance on the ground in Syria before further negotiations about the country's future are held to have finally over-ridden long-held regional differences, he said. Tajammu Al-Ezza brigade fighters fire a shell towards the forces of Syria's President Bashar al-Assad stationed at Salhab village, in the Hama countryside on Friday. Credit:Reuters "This regional group has forced those opposition groups and various factions fighting on the ground to fight under one umbrella," he said. "By creating this 'Army of Conquest' and by supporting it, having the Nusra Front as its main pillar and surrounded by the remnants of the Free Syrian Army as well as groups such as Ahrar al-Sham, Jaish al-Islam and others, this type of cooperation … has been a tremendous success." The model is now being copied in areas such as the Qalamoun – the mountain ranges between Lebanon's Bekaa Valley and Syria – where opposition forces this week began a fierce battle against the Islamic State, Syrian regime forces and the Hezbollah militants fighting alongside them. This is a three-sided conflict - on one side are the Nusra Front and its allies backed by Qatar,Turkey and Saudi Arabia, on the second side are Hezbollah, the Syrian regime and Iran and on the third side is the so-called Islamic State.

The key player on the Qalamon front is still the Nusra Front, along with its allies from the remnants of the Free Syrian Army, Mr Abou Zeid said. Tajammu Al-Ezza brigade fighters prepare shells to fire at the Assad forces on Friday. Credit:Reuters However there are reports that the group suffered heavy casualties in the Hezbollah-led offensive late this week. Hezbollah also acknowledged in a statement released on Friday that three of its soldiers had died in the fighting, although an opposition tweet put the toll as high as 30 fighters. A Syrian child evacuated on Friday from a fire in a compound housing Syrian refugees in the southern city of Sidon. Credit:AFP

"From the perspective of Hezbollah this area is very strategic because it is in close proximity from the Damascus Highway – to protect their supply lines it is very important for them to control the Qalamoun area," Mr Abou Zeid said. "You can see how much Hezbollah is mobilising the communities to join the fight … some villages [in the Bekaa area] have been almost emptied of their youngsters who have been deployed in order to launch such an attack." The Lebanese Army – loath to be drawn into any attack in the region after several of its soldiers were kidnapped in August last year – has also reportedly begun to mobilise in the area. The Nusra Front threatened to kill the soldiers if Hezbollah mounted an offensive in the Qalamoun area. "Everyone hopes the battle will remain on the Syrian part of the border and not spill over into the Lebanese territory," Mr Abou Zeid said, warning any overflow would have serious repercussions for the already fragile security situation in Lebanon.

Since it formed in March the Army of Conquest has grown in strength and capability, analysts say. But far from retreating, the Assad regime has reacted to its losses by carrying out hundreds of air strikes, barrel bombings, and chlorine attacks in rural Idlib, Hama, and Aleppo, Brookings Centre Doha visiting fellow Charles Lister wrote this week. The regime also launched ground offensives in eastern Damascus, Homs and in the mountains around Zabadani near the Lebanese border, he said. "Recent events have clearly tipped the psychological scales back into the opposition's favour," Mr Lister wrote, noting Assad's "severe manpower shortages are becoming more evident by the day". Hezbollah is stretched thin, he said, while even Iranian forces have begun withdrawing to the areas of Syria deemed to be the most important for regime survival.

The upsurge in fighting has only worsened the misery and danger for Syrian civilians, with the International Committee of the Red Cross warning the humanitarian situation has deteriorated sharply and the need for food, water and medical supplies is more urgent than ever. "The fighting is escalating in many parts of the country and more and more people are being forced to flee their homes. It is causing untold suffering," ICRC director of operations, Dominik Stillhart said after a two-day visit to Damascus. His comments follow the release of an Amnesty International report that warned civilians in the besieged city Aleppo are suffering "unthinkable atrocities", with many forced to try to live their lives underground in order to avoid attacks. The report found that both the Syrian Government and many of the armed opposition groups were perpetrating war crimes and crimes against humanity. "Civilians in opposition-controlled areas of Aleppo have been bombarded in their homes, hospitals, schools, public markets and places of worship in air attacks launched by government forces," Amnesty reported, resulting in thousands of deaths and injuries.

The majority of attacks have involved the use of 'barrel bombs' – large, improvised explosive devices dropped from helicopters, Amnesty reported. The bombs are made of oil barrels, fuel tanks or gas cylinders packed with explosives, fuel and metal fragments to increase their lethal effect. "The barrel bombs are the most miserable weapons. If they explode we know our bodies will be in pieces... [Also] we can see them coming. There is a minute of waiting to die," a 24-year-old woman said. Describing the aftermath of a barrel bomb attack, a 30-year-old factory worker told Amnesty International: "After the bombing, I saw children without heads, body parts everywhere. It was how I imagine hell to be." Follow FairfaxForeign on Twitter