The most significant breach of US defence department's computer network ever in 2008 occurred when an infected flash drive was used in a military laptop, a top Pentagon official has said.

The previously classified incident, which took place in 2008 in the Middle East, was disclosed by Deputy Defence Secretary William J Lynn in an article titled "Defending a New Domain" posted on Foreign Affairs magazine's website.

"The flash drive's malicious computer code, placed there by a foreign intelligence agency, uploaded itself onto a network run by the US Central Command," Lynn

"That code spread undetected on both classified and unclassified systems, establishing what amounted to a digital beachhead, from which data could be transferred to servers under foreign control," he added.

This significant breach served as an important wake-up call and led to a new Pentagon cyberdefence strategy known as Operation Buckshot Yankee.

"The 2008 intrusion that led to Operation Buckshot Yankee was not the only successful penetration. Adversaries have acquired thousands of files from US networks and from the networks of US allies and industry partners, including weapons blueprints, operational plans, and surveillance data," Lynn wrote.

But he provided no details on specific files lost or stolen in the attack.

After the incident, the Pentagon has built layered and robust defences around military networks and inaugurated the new US Cyber Command to integrate cyberdefence operations across the military, he said.

And over the past ten years, the frequency and sophistication of intrusions into US military networks have increased exponentially.

Every day, US military and civilian networks are probed thousands of times and scanned millions of times.

The Pentagon is now working with the Department of Homeland Security to protect government networks and critical infrastructure and with the United States' closest allies to expand these defences internationally.