BEIRUT - Islamist militants affiliated to al Qaeda seized the last remaining stronghold of Western-backed rebels in Syria's northwest province of Idlib on Saturday after days of fighting, rebels and a monitoring group said.Backed by other hardline Islamist groups, the Nusra Front are waging a major military campaign against the Syria Revolutionaries' Front led by Jamal Maarouf, a key figure in the armed opposition to President Bashar Assad, after accusing him of being corrupt and working for the West against them.The Nusra Front is al-Qaida's official affiliate in the Syrian civil war and was once one of the strongest insurgent groups fighting to topple Assad. But it has been overshadowed by the Islamic State, which has seized swathes of northern and eastern Syria and is now being targeted by US-led air strikesIn the past few days, the Nusra Front captured several villages in the Jabal al-Zawiya region of Idlib province and on Saturday it entered the village of Deir Sonbol, the stronghold of the Revolutionaries' Front, forcing Maarouf to pull out."Dozens of his fighters defected and joined Nusra, that is why the group won," Rami Abdulrahman, head of the Syrian Observatory for Human Rights told Reuters.A Nusra fighter confirmed the report, saying: "They left him because they knew he was wrong and delusional.""He left his fighters in the battle and pulled out. Last night, we heard them on the radio shouting 'Abu Khaled (Maarouf) escaped, Abu Khaled escaped'," he added.