By Khaled Yacoub Oweis and Erika Solomon
AMMAN/BEIRUT (Reuters) - Islamist fighters in Syria have joined forces to form what may be the biggest rebel army in the country, further undermining Western-backed military commanders and potentially challenging al Qaeda.
The announcement on Friday of a common leadership for the Islamic Front, an amalgam of six major Islamist groups which had earlier declared an intention to merge, coincided with accounts of a battle on the Turkish border between rival Islamists that ended with al Qaeda allies taking control of the town of Atma.
Factional fighting and fragmentation among those seeking to overthrow President Bashar al-Assad have hampered the revolt and the latest effort to unite has yet to show that it can result in effective coordination among groups which between them control large parts of Syria and some tens of thousands of fighters.
Gains by Assad since the United States held back from intervening following a poison gas attack on rebel territory in August have both hardened many rebels against the Western-backed Free Syrian Army (FSA), notionally charged with coordinating the war, and also galvanized some major formations to come together.
Ahmed Abu Eissa, leader of the Suqour al-Sham brigades, was named head of the Islamic Front, members of other groups said.
In announcing its formation in a video statement broadcast on Al Jazeera, Abu Eissa said: "This independent political, military and social formation aims to topple the Assad regime completely and build an Islamic state where the sovereignty of almighty God alone will be our reference and ruler."
His deputy was named as Abu Omar Hureitan of the Tawheed Brigades, another large Islamist formation which spearheaded a rebel offensive in Aleppo last year. Other groups joining the alliance were Ahrar al-Sham, Liwa al-Haq and the Islamic Army.
The Islamic Front may challenge the growing influence of two big rebel groups linked to al Qaeda that have drawn thousands of foreigners to fight a sectarian war pitting Sunni Muslim rebels against Assad's Alawites and their Shi'ite Iranian allies.
The Islamic State of Iraq and the Levant (ISIL) and the al-Nusra Front are not part of the Islamic Front, though they have cooperated with some of its component groups. While the new group's members want a Sunni Islamic state, most are more tolerant of other opinions than are those allied to al Qaeda.
"This is an extremely significant development, both in terms of symbolism and the military effect it will likely have on the ground," said analyst Charles Lister of IHS Jane's in London, who estimated the Front's forces might number at least 45,000.
"I am very enthusiastic, thrilled even," a member of the Suqour al-Sham brigades told Reuters, reflecting a general view among fighters after a series of military setbacks. "I believe this will be the start of a major turning point in Syria."
While divided world powers are trying to bring rebels and Syria's government together for talks, fighters on the ground have shown little interest in negotiating with Assad.
The Front would better represent forces on the ground and so undermine the exile leadership of the Syrian National Coalition, Lister said. And while he did not expect the Front to wage war on the Qaeda allies, its formation could draw Syrian Islamists away from ISIL and Nusra, where foreigners have a bigger role.
Highlighting factional rivalries, opposition activists said that ISIL had taken control of Atma, a crossing point for arms and other supplies to the rebels on the Turkish border, ousting a small, moderate Islamist group, Suqour al-Islam, which was under the command of the FSA general staff, based in Turkey.
Suqour al-Islam had fought other units loyal to the FSA earlier in the week and one local opposition activist, Abdallah al-Sheikh, suggested FSA commanders may have colluded with ISIL to dislodge Suqour al-Islam from Atma. ISIL fighters took Suqour al-Islam's leader and over 20 men prisoner, activists said.
Western powers that have backed the revolt since 2011 have fought shy of providing military aid to anti-Western groups. But secular factions, hamstrung by rivalries within Syria and Assad's opponents in exile, have been overshadowed by Islamists.
Gulf Arab states, notably Saudi Arabia and Qatar, have been arming and helping rebel groups, though the extent of foreign influence over the latest reorganization is unclear.
One source with connections to the Nusra Front said the group believed Qatar helped broker and finance the merger of the Islamic Front and that the Qaeda-linked groups were wary of the new alliance. He added: "I worry we are setting ourselves up for a risky division ... between the new Front and Nusra and ISIL."
(Writing by Alastair Macdonald)