A report by a National Security Agency (NSA) consultant has revealed that hackers have had "massive" access to Pentagon computer systems for the last three years, but officials are still unsure as to what data may have been compromised and who the intruders are.

The NSA has traced the attacks back to Russian origins but has had little response from the Russian government, prompting the US to file a formal complaint with the country.

It has named the intrusions "The Moonlight Maze", so-called because of the placing of numerous back doors which will allow later re-entry to the compromised systems, along with other tools that are set to re-route specific network traffic to Russia.

James Adams, the NSA consultant who revealed the hack, said it constituted the "most persistent and serious computer attack against the United States to date".

He added that the US had "disturbingly few clues" about who is behind the break-ins, how much data they have accessed and what other tools have been installed on government networks. But he said the revelation had sparked off the "largest cyber-intelligence investigation ever".

Only last month, Ronald Dick, director of the FBI's National Infrastructure Protection Center, revealed that between 1999 and 2000 there had been 102 detected computer intrusions into government systems.

But earlier than that, Michael Vatis, the FBI's computer security manager, said that the intruders had stolen "unclassified but still sensitive information about essential defence technical research matters".