Syrian rebels make concerted attack on al-Qaeda-linked fighters Aleppo is hit by bombs that destroyed a market and killed at least 35 in late December. The city has become a focus of the fighting against Isis Syrian rebels have launched what appears to be a co-ordinated armed assault against a transnational group with a hardline Islamist ideology and strong ties to al-Qaeda. Activists in Syria talked of clashes and violent jostling for control of positions as a newly formed rebel formation, the Syrian Revolutionaries Front (SRF), and its allies fought members of the Islamic State of Iraq and the Levant group, known by the acronym Isis. The fighting comes weeks before a planned conference in Geneva on Syria, which is in the midst of a civil war that is rapidly destroying infrastructure and has left more than 130,000 dead and 8m people, almost a third of the population, displaced. Late last week a gas pipeline in the Damascus countryside was sabotaged in an attack expected to cost millions of dollars to repair, the official Sana news agency reported. Isis has emerged as the most powerful of the forces fighting Bashar al-Assad's regime. However it has alienated many Syrians in opposition areas by seeking to impose a harsh, sectarian version of Islam on territories they control. Its rise has also complicated western policy. The US, UK and others sympathetic to the opposition cause are reluctant to funnel rebels any weapons that could end up in the hands of Isis or other like-minded extremist groups. Scores have died in two days of fighting, according to activists. Those reported to have been killed include a Tunisian commander of an Isis brigade. Photos posted to social media showed bloodied corpses and videos showed clashes. But reports from inside Syria remain patchy and the extent and success of the moderate rebels" advance against Isis remains unclear. Isis, made up fighters from across the Muslim world, has vowed to counterattack. "The organisation called Isis is distracting our revolutionaries and fighters with confrontations and secondary battles in order to exhaust their energy and power, as well as targeting the revolution icons, activists and officers in a way that serves directly the regime," said a statement issued by the newly formed SRF, a collection of rebel groups opposed to more Islamist factions. Followers have to give back their weapons, and declare their withdrawal from Isis. On Saturday Isis, which is already combating the US-backed government of Prime Minister Nouri Maliki in Iraq, claimed responsibility for a bombing two days" earlier of a neighbourhood in southern Beirut - a claim dismissed by some experts. John Kerry, US secretary of state, on Sunday called Isis "the most dangerous players" in Iraq and Syria. "The rise of these terrorists in the region and particularly in Syria, and through the fighting in Syria, is part of what is unleashing this instability in the rest of the region," he said at a press appearance in Israel. Other rebel groups, including both remnants of the western and Saudi-backed Free Syrian Army ones and those who share some of Isis" ideology, on Friday launched attacks on the group's checkpoints, camps and bases throughout parts of opposition-held northern Syria. The fighting was concentrated to the north and west of the northern city of Aleppo and most fighters acted under the SRF banner. Isis" killing of another rebel commander's brother apparently set off the violence, but tensions between it and more moderate groups have been simmering for months. "All rebel factions have acted against Isis at once - and had a surprise effect," said Cedric Labrousse, a researcher into the Arab world and Syria at Rennes university in France who has been closely following developments on the ground in Syria over the past few days. Now that effect is over and Isis has the element of surprise. Among major rebel groups, only the hardline Islamist Jabhat al-Nusra has declined to participate in the fight against Isis. It has urged a ceasefire, even though some members have joined the battle against the group. Even Liwa Tawhid, a major rebel group that tried to stay out of the fight, is being drawn in because of the death of one of its commanders at the hands of Isis, Mr Labrousse said. "There are so many forces that decided to clash with Isis that the situation will escalate," he said. As the fighting got under way on Friday the SRF, led by a commander named Jamal Maarouf, issued a scathing indictment of Isis, accusing it of acting in the interests of the Damascus regime and warning its fighters to lay down their weapons or to join other rebel groups. The statement also demanded that the many foreign fighters within Isis either leave Syria or join another rebel group. In recent weeks, many Syrian opposition activists have overtly accused Isis of being a tool of Mr Assad's government, noting that its members include those released by the regime from prison in 2011 and repeating allegations that the regime rarely launches air strikes on Isis bases. "They are currently used to create confusion and distort the image of the Syrian revolution," said Louay Mokdad, a spokesman for the Free Syrian Army. Isis supporters and intermediaries maintaining several social media accounts counter that rebels would have never been able to hold as much territory as they do without its determined, battle-hardened fighters.