Islamic State (Isis) claims it could purchase its first nuclear weapon within 12 months from Pakistan and then smuggle it into the US.
The claim was made in the group's propaganda magazine Dabiq, saying the situation is "more possible today than it was just one year ago".
According to the article, the terror group is expanding so rapidly it could afford to purchase the weapon.
British hostage John Cantile has apparently been writing for the magazine and has appeared in several propaganda videos and articles for the extremist group.
Cantile has been held hostage for more than two years.
In an article entitled "The Perfect Storm", it is claimed IS has billions of dollars in the bank and describes a "hypothetical operation" which involves buying a nuclear bomb "through weapons dealers with links to corrupt officials" in Pakistan.
The weapon could potentially be smuggled into North America by boat.
"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 a 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."
Anthony Glees, director of the Centre for Security and Intelligence Studies at the University of Buckingham, told the MailOnline, the idea of Pakistan selling nuclear weapons to IS "beggars belief. 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. 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."
"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."
In February this year, Cantile told his family to "let him go" and get on with their lives, after appearing on a propaganda video in which he ominously warned was the last in the series.
In a Dabiq article he thanked his relatives and fiancée 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."
Cantile was captured in Syria in November 2012 together with the now murdered US correspondent James Foley.