The Islamic State jihadist organization has recruited more than 6,000 new fighters since America began targeting the group with air strikes last month, according to the U.K.-based Syrian Observatory for Human Rights.
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At least 1,300 of the new recruits are said to be foreigners, who have joined IS from outside the swathes of Syria and Iraq that it controls.
The United States has launched some 165 air strikes on IS targets since early August. Other strikes have been carried out by the U.K and France, the latest a French attack on a logistics depot in Iraq on Friday.
A number of rebel commanders who oppose IS while continuing to fight the regime of Syrian president Bashar Assad have warned that the strikes are increasing local support for the jihadists.
IS runs aggressive social media-led recruitment campaigns in the Middle East and targets young Muslims in the West with slick English-language propaganda videos and digital magazines.
The bulk of the foreign fighters who have signed up in the past six weeks are aged between 15 and 20-years-old and have never been involved in a conflict before, according to Abdurrahman Saleh, a spokesman for the Islam Army, part of the Islamic Front rebel group.
The Islamic State receives approximately 20 new recruits a day in the town of north-west Syrian town al-Bab alone, Saleh told The Times newspaper. There are "hundreds" of other towns in Syria seeing a similar level of arrival, he added.
Many of the jihadists are believed to enter Syria across the porous border with Turkey after arriving at the popular tourist airport Antalya on budget airline flights.
Experienced fighters from areas like North Africa and Chechnya receive a monthly wage of up to $980 (3,570 shekels) a month, according to Hussam al-Marie, a spokesman for the Free Syrian Army. Professionals, such as doctors and surgeons, receive even more.