A cyber-attack that targeted Iran's oil ministry and main export terminal was caused by the most sophisticated computer worm yet developed, experts have warned.

The virus appears to have been directed primarily at a small number of organisations and individuals in Iran, the West Bank, Lebanon and the United Arab Emirates. This will inevitably raise suspicions that Israel or the US were involved in some way.

Analysts who have been decoding the computer worm, which is called W32.Flamer, have been unable to identify the source. But they say only a professional team working for several months could have been behind it.

The CrySys Laboratory, in Hungary, said: "The results of our technical analysis supports the hypothesis that [the worm] was developed by a government agency of a nation state with significant budget and effort, and it may be related to cyberwarfare activities."It is certainly the most sophisticated malware we [have] encountered. Arguably, it is the most complex malware ever found."