Powerful Syrian insurgent groups have rejected the authority of the opposition Syrian National Coalition.
The move badly damages efforts by Western-backed political exiles to forge a moderate rebel military force on the ground.
The 13 groups signed a statement calling for the opposition to President Bashar al-Assad to be reorganised under an Islamic framework and to be run only by groups fighting inside Syria.
The groups include at least three previously considered part of the coalition's military wing, the Free Syrian Army.
The signatories range from hardliners such as the al-Qaeda-linked Nusra Front and Ahrar al-Sham battalions to more moderate Islamist groups, such as the Tawheed Brigade and Islam Brigade.
In an online statement, political leader of the Tawheed Brigade Abdulaziz Salameh said: "These forces feel that all groups formed abroad without having returned to the country do not represent them, and they will not recognise them.
"Therefore, the National Coalition and its transitional government led by Ahmad Tumeh do not represent them and will not be recognised."
Western powers and their Gulf Arab allies had encouraged the coalition to lead a credible force within Syria under the FSA's banner and undercut Islamist militant groups piling into the conflict.
This effort to find a partner that the West and its allies could then back with weapons supplies could collapse if the rebel signatories hold their position. Some groups have previously backed away from statements with hardline forces.
Since the two-and a-half-year revolt against Mr Assad began, Syria's opposition forces have been riven with factionalism and rivalries.
There has also been tension between Islamist groups and those that support a secular vision for a post-Assad Syria.
UN weapons inspectors return to Damascus
United Nations chemical weapons inspectors have returned to Syria to continue investigating allegations of chemical weapons use.
A convoy of five UN cars carrying at least eight members of the team arrived at a central Damascus hotel this morning.
The inspectors confirmed last week that sarin gas was used in an attack in Damascus last month that killed hundreds of people.
President Assad's Western opponents said the inspectors' report left little doubt that his forces were to blame for the 21 August attack.
Syrian authorities denied the accusation, saying it made no sense for them to wage an attack with chemical weapons when their forces were making advances and while the inspectors were staying just a few kilometres away in the centre of the capital.
Russia has also said the inspectors' report did not provide irrefutable proof that Mr Assad's forces were responsible and that Syria had provided information it said showed rebels were behind the attack.