Fresh atrocities by Daesh

The Syrian Observatory for Human Rights confirmed the alliance had taken the Scientific Research Center, which was being used as a military barracks.

Observatory director Rami Abdel Rahman said that opened potential lines of attack against several other government-held neighbourhoods.

The alliance of rebels is one of at least two coalitions of opposition fighters that have begun a major operation against government-held districts in Aleppo in recent days.

A second coalition fighting in western Aleppo calls itself Ansar al-Sharia and includes al-Qaeda's local affiliate the al-Nusra Front.

Aleppo, once Syria's economic powerhouse, has been divided between the government in the west and rebels in the east since shortly after fighting there began in mid-2012.

A video shared online showed the capture of the Scientific Research Centre, with fighters from one of the groups raising the three-starred flag of the Syrian opposition.

The Observatory said Syrian government war planes had carried out extensive raids against the centre yesterday morning, prompting the rebels to evacuate parts of it.

The monitor said government forces had tried to recapture the centre overnight Friday without success.

Elsewhere in western Aleppo, clashes continued between rebels from Ansar al-Sharia and loyalists around the Zahra neighbourhood.

The group began an assault against Zahra and several other government-held districts Thursday night, advancing slightly before being pushed back Friday, as government planes carried out around 40 airstrikes.

The Observatory said at least 29 Ansar al-Sharia fighters were killed Friday, but had no details on deaths among government forces.

According to regional analysts what is making a substantial difference this time to the rebel offensive is that the rebels and their outside backers like Turkey, Qatar and Saudi Arabia, which were competing in the past, now seem to be pulling together jointly providing weapons and support. Over the last few weeks there have been persistent reports that the Syrian government with its forces seriously stretched, may be preparing to give up Aleppo in the north and Deraa in the south enabling them to defend the core areas of western Syria with limited manpower they have.

Also yesterday, the Observatory said the number of al-Nusra Front fighters killed a day earlier in an explosion in a mosque in northwestern Syria had gone up to 31.

An initial toll of 25 had been given for the blast, which hit a mosque during Ramadan prayers in the town of Ariha in Idlib province. Civilians were among the worshippers, but there was no immediate confirmation of civilian deaths. The Observatory said the blast appeared to have been caused by an explosive device placed at the mosque. A coalition including al-Nusra pushed government forces out of most of Idlib province earlier this year.

On another key frontline elsewhere in the country Syrian forces with their Lebanese Hezbollah allies are carrying out a major offensive against rebels holding the strategically important mountain town of Zabadani, according to Syrian state TV.

Zabadani, north-west of the capital, Damascus, is the last significant town held by Syrian rebels in an area close to the Lebanese border and has long been a thorn in the Syrian government's side since shortly after the country's crisis began in March 2011.

Taking Zabadani would tighten Hezbollah's grip on Syrian territories bordering Lebanon and would strengthen the Syrian government's control over of the Beirut-Damascus highway.

Hezbollah's al-Manar TV showed large clouds of smoke from explosions caused by artillery and air strikes and

Syrian state TV quoted a Syrian army source as saying its forces were "making headway on several fronts" and the rebels had suffered heavy casualties.

Raging for four years now the war in Syria has claimed quarter of a million lives and driven millions from their homes.