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A ransomware attack on a US natural gas facility meant a pipeline had to be shut down for two days, the US Department of Homeland Security (DHS) has said.

However, it did not name the facility or say when the attack happened.

A malicious link sent to staff at the facility eventually caused the shutdown "of the entire pipeline asset".

It was so severe in part because the organisation was not prepared for such an attack, the DHS statement said.

The incident was detailed , which revealed it to be a "spear-phishing" attack, in which individuals are sent fraudulent but believable scam messages.

That let the attacker into the company's IT network.

How did that shut down a pipeline?

Often, the "operational network" which runs computers in the factory is separated from the office IT - but not in this case, meaning the ransomware infection was allowed to spread.

Ransomware typically encrypts files on a victim's computer and demands payment before offering to unlock them again - although there is no guarantee that the cyber-criminals who develop such software will be true to their word.

A spate of ransomware attacks has troubled various US organisations recently - from local authorities to hospitals to a maritime base.

In the case of the natural gas facility, only one office was targeted, but others in different geographic locations were forced to close down, too.

The DHS said the affected organisation had not properly prepared for a cyber-attack of this kind - with its emergency plans being focused on all sorts of physical attacks instead.

"Consequently, emergency response exercises also failed to provide employees with decision-making experience in dealing with cyber-attacks," the department added.

All organisations, regardless of what sector they are in, should prepare for the possibility of a ransomware attack, said Carl Wearn, head of e-crime at cloud email firm Mimecast.

Businesses could do this "by implementing offline back-ups with a fall-back email and archiving facility, as a minimum" he said.