Istanbul , Turkey ( CNN ) -- A collection of some of Syria 's most powerful rebel brigades have rejected a Western-backed opposition group that announced the creation of an interim government in exile this month . The 13 rebel groups , led by the al Qaeda-linked al-Nusra Front , also called on supporters of the Syrian opposition to embrace Sharia law `` and make it the sole source of legislation . '' The move appeared to sideline the flagging National Syrian Coalition , which recently announced the formation of an interim government in exile led by Ahmad Tomeh , a dentist and dissident from the Syrian city of Deir Ezzor . In a statement released late Tuesday , the 13 rebel groups declared that `` the coalition and the putative government headed by Ahmad Tomeh does not represent or recognize us . '' Among the signatory rebel groups were Ahrar al Sham , Al-Tawheed and Suqoor al-Sham , three major fighting factions that control considerable amounts of territory in Syria . Members of the Syrian National Coalition contacted by CNN declined to comment , saying they were still working on an official response to the rebel statement . In a speech Tuesday to the United Nations , President Barack Obama pledged $ 340 million in additional assistance to Syria , while reiterating his support for what he described as the country 's `` moderate opposition . '' But moderate opposition groups appear to have suffered a series of setbacks this month as hard-line jihadi groups have pushed to grab more territory and resources . Last week , another al Qaeda-linked group drove rival rebels out of the strategically placed town of Azaz near the Turkish border . The group , whose ranks are bolstered by foreign jihadi militants , later accused the ousted rebels of being traitors for supporting democracy and for meeting previously with U.S. Sen. John McCain . Cash-strapped secular Syrian activists have been warning of the rise of Islamist groups in opposition-controlled parts of Syria . `` If anybody does a humanitarian project that is not under their ( the Islamists ' ) agenda , then they feel threatened , '' said one activist who runs humanitarian projects that help feed and house displaced families inside Syria . The activist asked not to be named , for fear of reprisal from Islamist groups . Turkey , an opponent of the Syrian government that has long allowed opposition fighters to travel freely back and forth across its long border with Syria , has recently begun sounding the alarm about extremist groups . `` This is a big security threat to us , and it directly concerns us , '' Turkish President Abdullah Gul told journalists in New York last weekend . `` These people may not have been radicals or terrorists at all in the beginning , but the conditions have brought them to this point , '' Gul said . `` People who would not harm a fly can find themselves in a position where they can mutually resort to atrocity and violence . '' It is not clear what effect the Islamist rebel groups ' announcement will have on Salim Idriss , the English-speaking former Syrian army general who now heads the Supreme Military Council of the rebel Free Syrian Army . Idriss has enjoyed the support of Western governments and officials such as McCain . He was filmed in April at the Turkish-Syrian border receiving a shipment of nonlethal assistance from U.S. State Department officials . Officials in the Supreme Military Council also declined to comment on the Islamist rebel announcement .
Syrian rebel brigades oppose Western-backed group that announced government in exile . The rebels have been fighting for more than a year to topple President Bashar al-Assad, who has ruled Syria since his father Hafez Assad took power after the death of his father, former leader and longtime Moderate opposition groups appear to have suffered a series of setbacks this month . Secular Syrian activists have been warning of the rise of Islamist groups .