( CNN ) Al Qaeda affiliate Jabhat al-Nusra has taken over key districts of Idlib suburbs in northwestern Syria , ousting U.S.-backed Syrian rebels from their strongholds and raising fears of their wider ascendance in northern Syria , monitors and activists say . The Islamist militant group , also known as al-Nusra Front , pushed the Syrian Revolutionary Front and the Hazm Movement out of the Idlib suburbs of Jabal al-Zawia and Jisr Alshugur , said the Syrian Observatory for Human Rights , a London-based monitoring group . One observer suggested the al-Nusra moves had taken about 70 % of the territory once held by moderates . But these moderates held little sway within military circles of the rebel movement , and they had influence mostly because of their links to the U.S. Like ISIS , al-Nusra Front is trying to establish an Islamic state , though primarily in Syria . It has emerged as one of the most effective groups fighting the Syrian regime , drawing on foreign fighters with combat experience in Iraq and elsewhere . The group has claimed hundreds of attacks in several cities , including suicide bombings , and is responsible for the deaths of `` numerous innocent Syrians , '' the U.S. State Department said . Islamist rivals in Syria find a common enemy in 'crusaders ' coalition . One focal point of the al-Nusra offensive was Syrian Revolutionary Front leader Jamaal Marouf , long seen as a moderate benefiting from U.S. assistance . He appeared in an online video saying that he had decided to withdraw from Jabal al-Zawia , his hometown , to stop further loss of life and that al-Nusra had killed more Syrians than the regime . However , there were reports the two sides were considering talks and a Sharia court may be used to arbitrate if Marouf personally appeared . There were also reports of defections from moderates to al-Nusra . One activist said many fighters were pressured by their families to defect as al-Nusra was popular in their hometowns . There are concerns that al-Nusra was moving to homogenize its control of the Idlib area , where it has long been dominant , and might target a key border crossing . The Syrian Observatory for Human Rights reported that al-Nusra fighters have mobilized in the town of Sarmada , near the Turkish-Syrian border crossing of Bab Al Hawa , leading to fears they may seek to take over that key border post . CNN 's Holly Yan contributed to this report .
U.S.-backed Turkish rebels have been ousted from their strongholds , monitors say . Reports say moderates , pressured by family , have defected to al-Nusra . There are concerns that ISIS might target a key crossing at the Syrian border . Like ISIS , al-Nusra is trying to establish an Islamic state , though primarily in U.S. .