BEIRUT - Powerful Syrian Islamist brigades, frustrated at the growing divisions among rebels, have joined forces in what they say is a "liberation front" to topple President Bashar Assad.Mistrust and miscommunication have been a feature of the rebel campaign against Assad. Differences over leadership, tactics and sources of funding have widened the rifts between largely autonomous brigades scattered across Syria.Ahrar al-Sham, a Salafist group which includes a large contingent of foreign fighters, withdrew, objecting to the killing of a Salafist leader killed by a rival rebel force.But rebel sources said talks were continuing to bring Ahrar al-Sham back, and leader of the new front, Ahmad al-Sheikh, said it was continuing to attract members."We have more than 40,000 fighters now and the numbers are growing because more brigades are expressing interest in joining," said Sheikh, known to his men as Abu Eissa.Accurate figures for the total rebel numbers are hard to establish but such a force could represent around half of Assad's armed opponents.Originally the group was called the Islamic Front to Liberate Syria. Brigade leaders voted to drop the word 'Islamic' but Islam remains a central element, Sheikh told Reuters."We are proud of our Islamism and we are Islamists. But we do not want to show it in a slogan because we might not live up to the responsibility of Islam," said Sheikh, who is also the head of the Sukour al-Sham Brigades. "But we want a state with Islamic reference and we are calling for it."Brigades in Damascus, Deir al-Zor, Aleppo, Idlib and Homs provinces have joined the front and logistical offices have been opened across Syria to facilitate coordination, Sheikh said.Attacks at checkpointsSince its formation, the front's fighters have been focused on attacking checkpoints as part of their attempt to push Assad's forces out of towns.On Tuesday fighters from the Sukour al-Sham (Hawks of Syria) seized the town of Maarat al Nuaman in Idlib province from government forces.The revolt against Assad began as peaceful protests calling for democracy and greater rights, but gradually turned to an armed struggle against his military machine, pitting the Sunni majority against the president and his minority Alawite sect.A lack of vision over how to topple Assad and divisions among opposition leaders encouraged more Syrians to take up arms. But splits soon surfaced among fighters, mirroring the disintegration among political opposition leaders.Many rebel leaders were angered that the head of the Free Syrian Army, the umbrella rebel group, was based in Turkey, saying it stripped him of any legitimacy among fighters who were dying inside the country.The belated move by Riad al-Asaad back into Syria last month has done little to change their stance."We are tired of paper tigers outside the country who have no link to the battlefield," said Sheikh, whose 16-year-old eldest son was killed in fighting in Idlib six months ago."We have gathered to unite the military work and we also have other agendas. We want to build the state of justice and give rights to its people after 40 years of oppression."Abu Eissa insists that all the Front's fighters were Syrian and none of the foreign fighters who have come to Syria will be allowed to join."We do not want anyone from outside, so that the revolution is not exploited and is not serving the agendas of others," he said.Their weapons are seized from attacks on Syrian army posts and some arms dealers inside and outside Syria, he said. Some European countries had promised they would soon recognise the front, he added.Tensions on the groundBut the move which is supposed to unite the rebels has also widened the rifts. Some in the FSA denounced the front and said that the emphasis on Islamic identity would worry minorities in the religiously mixed country.Some fighters also said the group receives funding from Gulf states which promote the same Islamist ideology - a reference to Saudi Arabia and Qatar - and also has better access to weapons coming through Turkey.They accused them of denying some of those arms to rebels from smaller groups fighting alongside them."We are fighting and getting killed but some do not even bother helping us. They just watch us as if we are not on the same front," said a fighter in a brigade composed of less than 500 insurgents.Sheikh said his front would maintain "brotherly relations" with all groups but fell short of offering support."Whoever wants to work with us is a brother and a son of the front and whoever wants to work under other wings in the interest of the revolution is also a brother for us. But the others who are in the camps (in Turkey), they do not have any acceptance among us."After more than a month of secret meetings, leaders of Islamist brigades - including the Farooq Brigade that operates mainly in Homs province and the heavyweight Sukour al-Sham brigade of Idlib - formed the "Front to Liberate Syria".The agreement is not the first which seeks to bring together disparate fighting groups and its Islamist emphasis has already alienated some other fighters.The growing role of the Islamist fighters and their battlefield prowess has also caused concern among Western powers as they weigh up how best to support the opposition forces arrayed against Assad.The new front does not include some groups which Western officials consider the most radical such as the Nusra Front, an affiliate of al-Qaida which has claimed responsibility for a series of devastating bombs in Damascus and Aleppo.