BEIRUT, Lebanon — As diplomats at the United Nations push for a peace conference to end Syria’s civil war, a collection of some of the country’s most powerful rebel groups have publicly abandoned the opposition’s political leaders, casting their lot with an affiliate of Al Qaeda.

As support for the Western-backed leadership has dwindled, a second, more extreme Al Qaeda group has carved out footholds across parts of Syria, frequently clashing with mainline rebels who accuse it of making the establishment of an Islamic state a priority over the fight to topple President Bashar al-Assad.

The fractured nature of the opposition, the rising radical Islamist character of some rebel fighters, and the increasing complexity of Syria’s battle lines have left the exile leadership with diminished clout inside the country and have raised the question of whether it could hold up its end of any agreement reached to end the war.

The deep differences between many of those fighting in Syria and the political leaders who have represented the opposition abroad spilled into the open late Tuesday, when 11 rebel groups issued a statement declaring that the opposition could be represented only by people who have “lived their troubles and shared in what they have sacrificed.”