A decision by the head of the jihadist Al-Nusra Front to pledge allegiance to Al-Qaeda chief Ayman al-Zawahiri has prompted unprecedented criticism from some of Syria's Islamist rebel brigades.
Until now, rebels had sought to bury their differences with Al-Nusra, reluctant to jeopardise ties with a force that is militarily superior to most of the country's rebel factions.
But an announcement this week by Al-Qaeda in Iraq, claiming Al-Nusra as part of its network, and a pledge of allegiance from Al-Nusra's chief to Zawahiri have prompted rare direct and public criticism.
"When we in Syria launched our jihad (holy war) against the sectarian regime... we did not do so for the sake of any allegiance to a man here or another there," the Syrian Islamic Liberation Front, an umbrella group of rebel brigades, said in a statement on Thursday.
It criticised the announcement from Al-Qaeda in Iraq, accusing the group of seeking to "impose a state on us without consulting us, led by an emir we did not choose or even hear of except through media outlets."
"Pledging allegiance to someone who does not understand our reality does not serve our people or nation," the group added.
Other more hardline rebel movements, including the powerful Salafist Ahrar al-Sham brigade which fights with the Syrian Islamic Front umbrella group, have been less willing to criticise Al-Nusra publicly.
But activists say the two groups have had strained relations, and the so-called "ideological godfather" of the Syrian Islamic Front criticised Al-Nusra's decision to pledge allegiance to Al-Qaeda.
"We protect the principles of Islam like the Islamic state, fighting in the name of God and his prophet Mohammed, Islamic law," Abu Baseer al-Tartusi said in a televised interview.
"But all reference to certain names that create a strong reaction around the world against the Syrian people must be avoided," he said.
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"You do not need to say that you belong to this name, or that I'm fighting for this banner... when you know that this will hurt the Syrian people and help the tyrant," he added, referring to President Bashar al-Assad.
Aron Lund, an expert on Syria's insurgency, said Al-Nusra's pledge of allegiance to the non-Syrian Zawahiri could backfire on the ground, where rebels are already suspicious of foreign fighters among Al-Nusra's ranks.
"This is a major public relations issue. Al-Nusra's own hardcore sympathisers may be happy about the allegiance to al-Zawahiri, but it may become more difficult for Al-Nusra to recruit new sympathisers," Lund told AFP.
Tensions between Al-Nusra and Islamist rebels have already boiled over once in the aftermath of the capture of Raqa, the first provincial capital to fall to opposition fighters.
In Tal al-Abyad in Raqa province, members of the Islamist Farouq brigade and other opposition fighters clashed with Al-Nusra for control after the withdrawal of regime troops.
Activists and observers have long warned that if the Assad regime falls, a battle between Al-Nusra and other rebel groups could follow. But Lund said the fighting in Raqa showed such conflict could erupt even before a government ouster.
"Raqa proves that day may be closer. With the regime out of the way, the battle shifted almost immediately," he said.
The latest developments "suggest to Syria's Islamists that Al-Nusra wants to go from the status of one military rebel force among others to the political leader of the uprising," Thomas Pierret, an expert on Islamism in Syria, told AFP.
"This is obviously unacceptable to them," the other groups, he added.
"For the first time, the Islamists have a tangible criticism to make on the place of Al-Qaeda in Syria," said Pierret, a lecturer at the University of Edinburgh.
"It can be summarised as: 'Who are you to proclaim an Islamic state when Assad hasn't yet fallen, and especially, who are you to proclaim yourself as leaders?'"