Increasingly paranoid Islamic State wary of spies on messaging-apps

"If you get onto the programs like WhatsApp and Telegram or others from Mosul, and get in touch with a person being tracked, the crusaders will start thinking about you ... assessing your importance and identifying the locations of the (Islamic State) centers by following you," said an article in the group's weekly newspaper, al-Naba, published online.

Twenty IS commanders have been killed this year including spokesman Abu Muhammad al-Adnani.

The message contained in al-Naba comes at a time when Iraqi forces, backed by US-lead coalition airstrikes, are battling IS militants encamped in the Iraqi city of Mosul, the biggest conurbation the extremist group controls.

In the magazine, IS members are further warned to switch off their mobile phones before entering any of the groups bases in order to avoid making the US-led coalition aware of their location.

"Switch off your phone after you finish your communication and beware of the greatest disobedience of all - switching it on when your are in one of the offices," it said. "As long as it has power, the phone is spying on you."

In Mosul, IS have enforced a crackdown on communication with the outside world as Iraqi forces advance on the city.

Satellite dishes are said to have been confiscated in the city to prevent residents watching progress made by the Iraqi army, while last month Iraqi intelligence officers claimed that IS militants had executed 42 people from local tribes in the area caught with SIM cards.

WhatsApp officially bars Islamic State supporters for a plethora of violations of its terms of service. However, actually identifying violators in private conversations is complicated by the fact that the Facebook-owned company implemented end-to-end encryption in the application earlier this year.

Telegram has a long history of anti-censorship struggles with different governments around the world and says that its policy is to block terrorist channels open to the public.

The company’s moto is “Taking back our right to privacy”.

Earlier this week, pro-Islamic State Telegram channels carried posts claiming responsibility for a knife attack that took place at Ohio State University, and detailed IS fighters in the Philippines' plans for expansion in southeast Asia.

This year, the UN also reported that IS fighters have used Telegram in order to set up online slave auctions, with posts carrying photos of captured Yazidi women, detailing their age, marital status, current location, and price.

Other encrypted apps used by the group are said to include Surespot and Threema.