The intruders stole "unclassified but still sensitive" information from US military computers, an FBI deputy assistant director, Michael Vatis, said.

Mr Vatis - also a director of the National Infrastructure Protection Centre - told a US Senate subcommittee on technology, terrorism and government information that the intrusions appeared to have originated in Russia.

Under attack: Pentagon's systems raided for over a year

The Los Angeles Times reported on Thursday that other officials had said some of the attacks had been traced to servers about 20 miles outside Moscow.

It said the pattern of attacks suggested that they might involve someone working in an office: they took place on weekdays between 0800 and 1700 Moscow time, but not during Russian holidays.

The newspaper said no classified computers were known to have been hacked and no networks damaged.

However, it quoted officials as saying that the US Government's unclassified networks contained large quantities of confidential and sensitive data of potential value to foreign governments, terrorists and private groups.

The LA Times said the hackers had also raided unclassified computer networks at Nasa, nuclear weapons and research laboratories, and numerous defence-related research organisations in universities and in the private sector.

Russian denial

Russia's SVR foreign intelligence service said it was "unlikely" to have been involved.

Spokesman Boris Labusov told the Interfax news agency: "That would be clumsy work for Russia's secret services."

He said the hacking operation might have been masterminded by the "secret services of third countries that used Moscow addresses to hide their tracks".

The Pentagon on Thursday set up a "cyber-war" centre to defend the US from hackers, and plot ways to attack enemy computer networks.

The chairman of the joint chiefs of staff, General Henry Shelton, said the move acknowledged that it was critical to "protect our systems and potentially get inside an adversary's system and affect his decision".

Gen Shelton also admitted that the US had tried to attack Yugoslav computer networks during the Kosovo war.