An article, apparently penned by British hostage John Cantlie for the terror group’s magazine, Dabiq, says the scenario is 'more possible today than it was just one year ago'.
Cantlie, a photojournalist who has been held captive for more than two years, has appeared in multiple propaganda videos and articles for the extremist group.
In an article entitled 'The Perfect Storm', it is claimed ISIS has billions of dollars in the bank and describes a 'hypothetical operation' which involves it buying a nuclear bomb 'through weapons dealers with links to corrupt officials' in Pakistan, Daily Mail reports.
John Cantlie has been held captive for more than two years
It describes how the device could be smuggled into North America overland and by boat.
'Perhaps such a scenario is far-fetched but it’s the sum of all fears for western intelligence agencies and it’s infinitely more possible today than it was just one year ago,' it adds.
It says 'if not a nuke', a few tonnes of explosives could be used instead.
'The Islamic State make no secret of the fact they have every intention of attacking America on its home soil and they’re not going to mince about with two mujāhidīn (“jihadis”) taking down a dozen casualties if it originates from the caliphate,' it reads.
'They’ll be looking to do something big, something that would make any past operation look like a squirrel shoot, and the more groups that pledge allegiance the more possible it becomes to pull off something truly epic.
The article ends with the chilling warning: 'As the territory of the Islamic State crosses from one border to another like a wildfire that is burning out of control, it’ll be only a matter of time before the Islamic State reaches the western world.'
ISIS is swaggering and posturing. It is taunting both us in the West but also other Middle Eastern states and embarrassing Pakistan at the same time.
Anthony Glees, director of the Centre for Security and Intelligence Studies at the University of Buckingham, said the idea of Pakistan selling nuclear weapons to ISIS is 'beggars belief'.
"'He's either been brainwashed by ISIS and by fear of what they can do to him or doing his best to stay alive. It's an enigma.'"
'It would be suicidal for Pakistan to supply them and suicidal for ISIS to seek to acquire them - it would lead to immediate military intervention,' he told MailOnline.
'However it is possible that ISIS might try to acquire nukes from somewhere, but if so, all western intelligence agencies would be on the highest alert to prevent this.
'As for getting non-nuclear explosives, well that would indeed not be hard. But they're not nukes, I can't believe ISIS could acquire nukes.
'But the bottom line is that ISIS is strengthening its stranglehold on Iraq and Syria,' Professor Glees explained. 'That is desperately bad news for us.'
In February, Cantlie told his family to ‘let him go’ and ‘get on with their lives’ - days after appearing on a propaganda video which he ominously warned was the last in the series.
In a Dabiq article he thanked his relatives and fiancee for their ‘tireless efforts’ campaigning for his release. However, he adds: ‘Let it go. Leave it be and get on with your lives, all of you’.
He was captured in Syria in November 2012 together with the now murdered US correspondent James Foley.
Professor Glees added: 'As for Cantlie, it is impossible for us in the free West to appreciate the pressure that has been put on him.
'He's either been brainwashed by ISIS and by fear of what they can do to him or doing his best to stay alive. It's an enigma.'
'ISIS is swaggering and posturing. It is taunting both us in the West but also other Middle Eastern states and embarrassing Pakistan at the same time.'