Eleven Islamist rebel groups in Syria have announced they do not recognize the authority of the main opposition alliance, the National Coalition, according to BBC News.

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.

In a video statement published online, 11 of the most powerful Islamist groups declared that they rejected the idea that opposition leadership could come from any organization based outside Syria that was not working closely with those on the inside.

"These forces feel that all groups formed abroad without having returned to the country do not represent them, and they will not recognize them," said the statement by Abdulaziz Salameh, the political leader of the Tawheed Brigade, according to Reuters. "Therefore the National Coalition and its transitional government led by Ahmad Tumeh do not represent them and will not be recognized."

The Istanbul-based Western-backed National Coalition was formed in November 2012 and is recognized by more than 100 countries as a legitimate representative of the Syrian opposition.

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".

Charles Lister of IHS Jane's Terrorism and Insurgency Centre said the three moderate Islamist groups which signed the statement - the Tawheed Brigade, the Islam Brigade and Suqor al-Sham - had represented SNC's main rebel presence on the ground.

"The inclusion of the core of the SNC force...effectively depletes the SNC's armed wing, the Syrian Military Council," Reuters quoted Lister as saying. "It is likely that the moderate Islamist coalition has ceased to exist as a single organization structure."