BEIRUT - Powerful Syrian Islamist brigades, frustrated at the growing divisions among rebels, have joined forces in what they say is a "liberation front" to topple President Bashar Assad.

Mistrust and miscommunication have been a feature of the rebel campaign against Assad. Differences over leadership, tactics and sources of funding have widened the rifts between largely autonomous brigades scattered across Syria.

After more than a month of secret meetings, leaders of Islamist brigades - including the Farooq Brigade that operates mainly in Homs province and the heavyweight Sukour al-Sham brigade of Idlib - formed the "Front to Liberate Syria".

The agreement is not the first which seeks to bring together disparate fighting groups and its Islamist emphasis has already alienated some other fighters.

The growing role of the Islamist fighters and their battlefield prowess has also caused concern among Western powers as they weigh up how best to support the opposition forces arrayed against Assad.

The new front does not include some groups which Western officials consider the most radical such as the Nusra Front, an affiliate of al-Qaida which has claimed responsibility for a series of devastating bombs in Damascus and Aleppo.

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