(Reuters) - The most feared and effective rebel group battling President Bashar al-Assad, the Islamist Nusra Front, is being eclipsed by a more radical jihadi force whose aims go far beyond overthrowing the Syrian leader.

Al Qaeda's Iraq-based wing, which nurtured Nusra in the early stages of the rebellion against Assad, has moved in and sidelined the organization, Nusra sources and other rebels say.