BEIRUT - A group of powerful rebel units have rejected the authority of the Western-backed Syrian opposition leadership abroad and called for it to be reorganized under an Islamic framework, according to a video statement posted on the internet.At least 13 rebel factions were said to have endorsed the statement, including the al-Qaida-linked Nusra Front and the powerful Islamist battalions Ahrar Asham and the Tawheed Brigade."These forces call on all military and civilian forces to unite under a clear Islamic framework based on Sharia law (Islamic law), which should be the sole source of legislation," the statement said.The man reading the statement was not immediately identifiable to Reuters, but the video was posted on several of the social media pages belonging to groups said to have signed on to it, such as the Tawheed brigade.Whatever their ideological background, many Syrian opposition groups inside Syria are skeptical and resentful of the Western and Gulf Arab-backed National Coalition, an umbrella organization for opposition groups abroad.Critics accuse it of not being transparent with funding and its political processes and of being out of touch with those living inside Syria, where more than 100,000 people have been killed and swathes of territory have been destroyed by combat and shelling."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 an elderly man reading the statement on film."Therefore the National Coalition and its transitional government led by Ahmad Tumeh do not represent it and will not be recognized."Since the 2-1/2-year revolt against President Bashar Assad began, Syria's opposition forces have been riven with factionalism and rivalries. There have also been tensions between Islamist groups and those that support a secular vision for a post-Assad Syria.Islamist forces have grown in power since the transformation of the rising from a peaceful protest movement into an armed insurgency combating a fierce crackdown by the government.