Militant Islamist fighters take part in a military parade along the streets of Syria’s eastern Raqqa province, June 30, 2014. Reuters

The huge influx of foreign fighters to Syria and Iraq has reportedly made Islamic State militants nervous. Militants have tightened recruitment requirements and security checks for westerners prior to arriving to their stronghold, as ISIS fears spies may be disguised as fighters and infiltrate the group, Asharq al-Awsat reported.

A recent U.N. report said foreigners were joining the group formerly known as ISIS on an “unprecedented scale.” So many people have joined, it seems even ISIS is concerned it won’t be able to weed out spies. The Asharq al-Awsat report said the group recently released a detailed list of requirements for foreign fighters in addition to advice on how to make the trip to Syria without getting caught.

International Business Times previously reported westerners hoping to join ISIS were vetted before being sent to one of the militants’ strongholds. Part of that vetting process entailed knowing the right extremist enthusiasts who could vouch for a candidate's commitment to the ideology. When a westerner decided to join, he had to find someone who could facilitate the jihadi journey -- a mentor. For many, that mentor could be found online or sometimes through supporters in local communities.

“If you know somebody, they’ll probably get in touch with someone from your place, whether you're American, Canadian or British,” Mubin Shaikh, a former extremist recruiter who operated from his hometown of Toronto before becoming a national security operative in Canada recently told IBTimes. “They’ll ask you what area you’re from, what scholars you know.”

The vetting process is not new, but requirements have become much stricter and westerners must now have a recommendation from one prominent sheik already known to ISIS, according to Asharq al-Awsat. The group has also issued strict guidelines on how the recruits should dress, what they should bring with them to Syria and explicitly stated that potential recruits should not inform anyone of their plans until after they’ve arrived and completed their initial training and “ribaat” (mercenary duties).

“Your clothes should not identify your religion and you should remain silent and not talk too much,” the guidelines state, according to Asharq Al-Awsat. “The only thing people should know about you is that you are a normal traveler.”

For those unable to get a sheik’s recommendation, all hope is not yet lost. The group will conduct its own security and background checks once a potential recruit has expressed interest in joining.

“For those who don’t have sponsorship, you should be patient with the brothers’ security checks, and some parties completely reject accepting recruits who do not have recommendations while others accept but only after you pass strict tests,” the guidelines state.

A member of ISIS who has been accused of being a spy rarely lives to tell the tale. Last month, the group reportedly executed one of its prosecutors in Deir el-Zour, who went by the name of Abu Abdullah al-Kuwaiti, after he was accused of being an operative for the CIA. Journalists operating near ISIS often face the same fate as ISIS has a history of accusing most media personnel of spying.