File Photo: Syrian Forces in Aleppo. (Reuters Photo)

Two alliances of Syrian rebels battled to advance in government-held western Aleppo on Saturday, seizing an army barracks in one district but losing ground in others, in some of the fiercest fighting in the city since the conflict began.US-led coalition forces, meanwhile, said they carried out a series of 16 airstrikes against the Islamic State group in their Syrian stronghold of Raqa, one of their biggest assaults on the extremists.The announcement came hours after IS released a video showing teenage members executing 25 Syrian soldiers in an amphitheatre in the ancient ruins of Palmyra.Elsewhere, Syrian government forces launched a major operation to recapture Zabadani, the last opposition-held town in the Qalamun region near Damascus.The Syrian Observatory for Human Rights, a Britain-based monitor, reported heavy clashes in several parts of western Aleppo, Syria's former economic powerhouse.It said fighters from the Conquest of Aleppo alliance of mostly moderate forces had captured a former research centre being used as a military barracks, opening up lines of attack against several nearby regime-held districts.On Saturday, clashes raged around the complex as regime forces backed by airstrikes sought to recapture it, marking some of the worst violence in Aleppo since the Syrian conflict arrived in the northern city in mid-2012.Conquest of Aleppo and a second rebel grouping, Ansar al-Sharia which is dominated by Islamists and Al-Qaeda's local affiliate Al-Nusra Front, began operations against government-held districts of Aleppo this week.The attacks have prompted heavy fighting and bombardments, with rebels firing hundreds of rockets and the government launching dozens of air strikes.The assault began on Thursday, when Ansar al-Sharia attacked several neighbourhoods including Zahra, where an air force intelligence facility is based.The coalition initially made progress, but was pushed back by government forces and heavy airstrikes that killed 29 rebels on Friday night, the Observatory said.Thomas Pierret, a professor at the University of Edinburgh and a Syria expert, said the latest fighting could prove significant.The recent rebel progress in and around western Aleppo "is the first real progress by the rebels in Aleppo since... July 2013," he told AFP.Control of the city has been largely divided between the government in the west and rebels in the east since shortly after fighting began there three years ago.Front lines have remained fairly static since then, though both sides have at times sought to encircle the other by advancing in the countryside around the city.Pierret said the loss of the barracks left the regime's hold on west Aleppo "more vulnerable".But he said progress would not be easy for the rebels because the city's tall buildings are ideal for regime snipers and its avenues are wide enough for government tanks.Elsewhere, IS on Saturday released a video showing the execution of 25 Syrian soldiers in the ancient amphitheatre in the city of Palmyra.The killings were first reported shortly after IS seized the ancient desert city on May 21, and were some of the more than 200 executions it carried out in the area around that time.The video showed young boys and teenagers shooting the kneeling soldiers dead in front of a large IS black-and-white flag, as a crowd of men and some children watched.Also on Saturday, US-led coalition forces carried out a series of 16 airstrikes against IS in its Syrian stronghold of Raqa, one of the biggest assaults on the extremists so far."The significant airstrikes tonight were executed to deny Daesh the ability to move military capabilities throughout Syria and into Iraq," spokesman Lieutenant Colonel Thomas Gilleran said in a statement, using another acronym for IS."This was one of the largest deliberate engagements we have conducted to date in Syria and it will have debilitating effects on Daesh's ability to move from Raqa."In the Qalamun region meanwhile, Syrian government forces backed by Lebanon's Shiite movement Hezbollah began a battle to capture the last rebel-held town in the area north of Damascus on the Lebanese border."Army units supported by the Lebanese resistance this morning launched a major operation in the Zabadani region in Damascus province," Syrian state television announced.The Observatory said at least 22 barrel bombs were dropped in the area on Saturday alone as heavy fighting was underway between regime forces and their allies and rebels in the area of Zabadani, one of the first towns to fall to the opposition in 2012.The news came a day after rebels attacked an army outpost at the entrance to the town, which the regime responded to with some 90 air raids using barrel bombs and missiles.Regime troops backed by Hezbollah have also recaptured most of the Qalamun region, which was once an opposition bastion.