text: The U.S. and her allies are at an ever growing risk of a systemic cyber-assault, with energy infrastructure likely to be hacker's prime target. The stark warning comes from General Keith Alexander, the retired four-star general and former chief of the National Security Agency. 'The greatest risk is a catastrophic attack on the energy infrastructure. We are not prepared for that,' he warned. Scroll down for video . Former NSA chief General Keith Alexander has warned that the U.S. and her allies are at an ever growing risk of a systemic cyber-assault, with energy infrastructure likely to be hacker's prime target . Alexander, who led the U.S. battle against cyber-threats for much of the last decade, was speaking at a private dinner held by IHS CERAWeek in Texas last week, reports the Daily Telegraph. He envisioned a worst case-scenario where hackers targeted oil refineries, power stations, and the electric grid. The payments nexus of the major banks could also be paralysed he warned. 'We need something like an integrated air-defence system for the whole energy sector,' he said. Alexander listed five countries able to conduct cyber-warfare at the highest level: the US, UK, Israel, Russia and Iran. The current NSA chief Michael Rogers testified late last year that China is capable of cyber-attacks that could cause 'catastrophic failures' of the water system or the electricity grid. There is no suggestion that China has an intention to use its power to damage US infrastructure. NSA officials are less confident that Iran will show self-restraint. Alexander also voice his concern about the hacking threat from a terrorist organization such as ISIS. He said that the U.S. and her allies had failed to check the advance of ISIS and its expanding network of franchises across the Middle East.
target: General Keith Alexander has warned that the U.S. and her allies are at an ever growing risk of a systemic cyber-assault .
  He predicts that energy infrastructure would be hacker's prime target .
  'We are not prepared for that,' he warned in Texas last week .
  Also voiced his concerns about the hacking threat from a terrorist organization such as ISIS .