ISIS have advised would-be jihadists to pretend to be Christians to avoid detection in a disturbing guide on how to carry out terror attacks.

The 64-page manual, entitled "Safety and Security Guidelines for Lone Wolf Mujahideen," has been translated into English and published online. It advises militants to trim their beards, wear western-style aftershave and pretend to be Christians so plans aren't thwarted by security officials.

"No doubt that today, at the era of the lone wolves, brothers in the West need to know some important things about safety in order to ensure success in their operations," the document's introduction reads.

"We thought a lot of non-Arabic speaking brothers would find it interesting and may apply it in their blessed operations."

It isn't the first time ISIS militants have been encouraged to disguise themselves as Westerners or non-Muslims in order to avoid suspision. Taqiyya is an Islamic practice which permits a Muslim to deny his faith or live in an otherwise sinful way in order to avoid detection.

"If you can avoid having a beard, wearing qamis [Islamic tunics], using miswak [teeth cleaning twigs] and having a booklet of dhikr [prayers and devotional acts] with you, it's better. It is permissible for you to wear a necklace showing a Christian cross," the manual reads.

"As you know, Christians – or even atheist Westerners with Christian background – wear crosses on their necklaces. But don't wear a cross necklace if you have a Muslim name on your passport, as that may look strange," the guide adds.

This particular manual was initially published by al-Qaeda but has been updated by ISIS to include advice on avoiding detection as well the suggestion of potential targets.

The 12 chapters within the manual give instructions on which encrypted mobile apps to use and how to go online without being detected.

It is one of several online ISIS terror guides. Others have included advice on how to make car bombs and how to cross into Syria via Turkey undetected.