Moderate Syrian rebels have made a renewed call for military support from the West just days after launching a new front against a powerful al-Qa'ida-linked group - with whom they had previously allied in fighting President Bashar al-Assad.

Fierce clashes across northern Syria over the past five days have seen the Islamic State of Iraq and the Levant (ISIS) routed from a number of its strongholds by fighters affiliated with the Western backed opposition.

The civil war within a civil war intensified in several cities after ISIS fighters were forced from their headquarters in the provincial capital of Raqqa - long seen as a bastion of strength for the group. There were unconfirmed reports that close to 300 prisoners were freed from the Raqqa HQ, while clashes were also taking place at the group’s headquarters in the frontline city of Aleppo.

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By Wednesday afternoon, activists reported that ISIS had been forced from the towns of Kafr Nabl , Kafr Roma, Maaret al Noman, Maaret Herma, Tal Mennes, Jarjnaz, Babila, Maasran, Maaret Debseh, Khan al Sebl, Menbej, Al Bab, Binish.

"The Rubicon has been crossed. This is all out war," Oubai Shahbandar, a senior advisor to the opposition who is in close contact with the Free Syrian Army, told The Independent.

In a sign that fighting was likely to continue for the foreseeable future, a spokesman for ISIS released a statement on Tuesday evening, calling everyone associated with the Syrian Opposition Coalition (SOC) - the official body of the opposition-in-exile, based in Istanbul - a "legitimate target."

The confrontation between the more moderate rebels and ISIS - both of whom are simultaneously fighting to remove Assad from power - was seen by many as inevitable due to the radically different visions they both hold of what a future Syria would look like.

Moderate rebel groups fighting under the banner of the Free Syrian Army (a blanket term for rebel brigades affiliated with the Syrian opposition-in-exile) have long called for military support from the West to aid its fight against Assad. But Western countries have been reluctant to assist, in part due to concerns that any arms they provide could fall into the hands of extremists.

The rise of ISIS - which is made comprised predominantly of foreign fighters - and other al-Qa'ida-linked groups has led to speculation that the West may prefer Assad to stay in power in order to battle extremists, who may one day target Western cities.

But the opposition claim that this concern is no longer valid since they began to fight ISIS, and have renewed their call for military support.

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"The events of the last few days prove that there is a viable choice in Syria when it comes to fighting extremism," Shahbandar said. "The Assad tactic is to represent the conflict as a choice between the regime and al-Qaida. This uprising against al-Qaida discredits this argument."

The battle in northern Syria comes at a crucial time for Syria's opposition. Later this month, its representatives are due to sit down with the Syrian government at peace talks in Geneva aimed at bringing an end to the nearly three-year-old civil war. On Sunday, in order to prepare for the Geneva meeting, the SOC will meet in Paris with ministers from the Friends of Syria grouping - the 11 nations who back the opposition, including the US, UK and France.

Mr Shahbandar said representatives from the Supreme Military Council (SMC) – the military wing of the Syrian opposition – will use the meeting to push for military support to battle al-Qa'ida and the Assad regime.

"There has to be a redoubled effort to ensure that the moderates that are fighting extremist have the means to sustain themselves. The reason why al-Qa'ida was able to expand so quickly in the first place was because the balance of power was skewed against us."

In recent months, ISIS has faced mounting opposition from civilians living in the areas it controls. Protests have been held in Aleppo and Raqqa against the harsh rule of the group, which has meted out punishments for the failure of citizens to adhere to sharia law.

Tensions between moderate rebels and ISIS have been growing since they first clashed in September, in the border city of Azaz. Minor skirmishes have been commonplace since then, but Friday's assault marked the first time a coordinated rebel force has hit back against the well-organised ISIS fighters.

The majority of attacks on ISIS positions were carried out by two newly formed rebel brigades, the Syria Revolutionaries' Front (SRF) and the Mujahideen Army. The former was set up by the Supreme Military Council specifically to combat al-Qai'da, and is made up of approximately 13,000 fighters, according to an SMC spokesman.

The clashes were also notable for the involvement, albeit minor, of some factions within the Islamic Front - the largest and most powerful rebel coalition in Syria.

Whether the moderate rebels can hold on to the territory regained from ISIS remains to be seen, but analysts agree the move against the group is significant.

"The attacks [...] seem to have hit the Islamic State pretty badly" said Aron Lund, editor of the Carnegie Endowment's Syria in Crisis. "They lost territory and fighters, many defected or surrendered. Some groups that had recently joined them suddenly decided they were neutral and wouldn't fight for the group after all. There is a lot of talk about how foreigners were forced to seek refuge with Jabhat al-Nusra, the al-Qa'ida faction, which is playing the role of a sort of neutral mediator."

"That doesn't mean the Islamic State is finished, by any means," he added. "They still have some areas in the north-west and much further east. They could regroup now that the element of surprise is gone. Maybe they can recapture some areas, maybe not. But if they're going to be at war with FSA-style groups and even some of the big Islamist factions, that's going to take up most of their time, whether or not they are able to turn the tide."

Video: Clashes in Syria