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The statement was read out by the political leader of Liwa al-Tawhid
Eleven Islamist rebel groups in Syria have announced they do not recognise the authority of the main opposition alliance, the National Coalition.
A joint statement says: "All groups formed abroad without having returned to the country do not represent us."
They also call for the opposition to unite under an "Islamic framework".
Islamist rebel forces have become increasing prominent in the conflict in Syria, and they are believed to command tens of thousands of fighters.
The signatories include members of the Free Syrian Army as well as more radical Islamists - among them the powerful al-Nusra Front, which has links to al-Qaeda.
It comes amid fighting on the ground between the Islamic State of Iraq and the Levant (ISIS), an offshoot of al-Qaeda, and more moderate rebel forces, especially in areas along Syria's northern and eastern borders.
'Unite under Sharia'
In a video statement published online on Tuesday, 11 of the most powerful Islamist groups declared that they rejected the idea that opposition leadership could come from any organisation based outside Syria that was not working closely with those on the inside.
The Istanbul-based Western-backed National Coalition was formed in November 2012 and is recognised by more than 100 countries as a legitimate representative of the Syrian opposition.
"These forces believe that it they are most legitimately represented by those who have lived the same experience and shared in the same sacrifice of their honest sons."
"Therefore the National Coalition and its transitional government led by Ahmed Tomeh do not represent it and will not be recognised."
Mr Tomeh was appointed last month to form an administration to govern rebel-held areas of Syria and co-ordinate the provision basic services and supplies.
The statement also called on "all military and civilian forces to unite under a clear Islamic framework based on Sharia [Islamic law], which should be the sole source of legislation".
It urged members of the opposition to "reject division" and put "the interest of the Ummah [Islamic nation] over the interest of each group".
The BBC's Kevin Connolly in Beirut says the declaration reflects two growing trends.
One is the increasing influence of Islamist rebel movements within the coalition fighting President Bashar al-Assad; the other is the "Islamisation" of some of the other groups, including members of the Free Syrian Army (FSA), our correspondent says.
The statement is an attempt to shift the political balance of power in favour of those doing the fighting on the ground and is also an indication that jihadist groups like the al-Nusra Front are very much centre stage in the conflict, an awkward reality for Western powers, he adds.
Charles Lister of IHS Jane's Terrorism and Insurgency Centre said the three moderate Islamist groups which signed the statement - Liwa al-Tawhid, Liwa al-Islam and Suqur al-Sham - had represented the National Coalition's main rebel presence on the ground in Syria.
"The inclusion of the core of [the National Coalition's] force... effectively depletes [its] armed wing, the Supreme Military Council," he told the Reuters news agency. "It is likely that the moderate Islamist coalition has ceased to exist as a single organisation structure."
In a separate development on Wednesday, United Nations inspectors returned to Syria to continue their investigation into the alleged use of chemical weapons during the conflict.
Last week, they confirmed that the nerve agent sarin was used in an attack on several suburbs of Damascus on 21 August, in which hundreds of people were killed.
They plan to visit three further sites - Khan al-Assal, Sheikh Maqsoud and Saraqeb - where smaller scale chemical attacks are alleged to have taken place earlier this year.