The FBI is investigating an unusually powerful denial of service attack that crippled nine of the 13 servers that manage global internet traffic.

The attack, which happened on Monday, lasted an hour and meant that it took much longer than usual for users to log-on to web pages for the first time.

According to Associated Press, the FBI is concerned because the attack appeared to be the "the largest and most sophisticated assault on the servers in the history of the internet".

Experts explained that the attack transmitted data to each targeted root server at 30 to 40 times the normal amount, and that just one additional failure would have disrupted email messages and browsing across many parts of the web.

Seven of the servers failed to respond to legitimate network traffic, and two failed intermittently during the attack.

Administrators used defensive measures to limit the damage, and the attack suddenly stopped an hour after it began.

The 13 computers are spread across the globe as a precaution against physical disasters, and are operated by US government agencies, universities, corporations and private organisations.

Richard Clarke, top cyber-security adviser under the Bush administration, and head of the protection board, has warned for months that an attack against the internet's root servers could cause severe disruption.