What does it take to shut down an entire city? A nuclear bomb? Thousands of heavily armed troops? Or an epidemic?

All these are possible, but there is another way. For the head of the British security and intelligence organisation, just one click of a button could be enough to shut down cities.

Robert Hannigan, the United Kingdom's Government Communications Headquarters (GCHQ) chief, warned that terrorists and rogue states are building up their technical capabilities to paralyse urban areas in an instant.

In a rare public appearance at the Cheltenham Science festival, the British intelligence chief said terror groups are planning to shut down big cities like London using the so-called "Internet of things"—the increasing online connectivity of common things like cars and household appliances.

"There are certainly states and groups with the intent to do it, terrorist groups, for example, who have no threshold when it comes to the loss of life," Hannigan told participants of the festival, as quoted by The Telegraph.

"We're not quite there yet, but as the world becomes ever more connected that will become a greater risk," he added.

The head of the GCHQ further said that rogue states are actually developing the kind of cyber programmes that can be used to attack major cities. Terrorist groups, for their part, are looking at acquiring this technology.

"At some stage they will get the capability," he said.

With this in mind, Hannigan said intelligence services should step up surveillance of the Internet, saying that the U.K. has intercepted at least seven cyberattacks for the past 18 months.

In an opinion piece on Jihad Watch, columnist Christine Williams said the statements made by Hannigan do not bode well for the whole world in terms of security.

Williams said this means that terror groups like the Islamic State have already turned into "killing machines" that are "destructive without boundaries."

She further urged the United States to step up its vigilance against terrorism following the British intelligence chief's warning.

"In the words of al-Qaida leader Ayman al-Zawahiri: 'The first matter is striking the West and specifically America in its own home, and attacking their interests that are spread everywhere'," Williams said.