Online attacks cost Pentagon US$100 million

Speaking at a cybersecurity conference in Nebraska, Chilton said the money was being spent on computer equipment, contractors and manpower to clean up after external attacks and internal mistakes.

"The important thing is that we recognise that we are under assault from the least sophisticated — what I would say the bored teenager — all the way up to the sophisticated nation-state, with some pretty criminal elements sandwiched in-between," Chilton told Associated Press.

"This is indeed our big challenge, as we think about how to defend it."

Chilton declined to say what percentage of the attacks came from outside the military’s systems, or to comment on the likelihood that some attempts were being made by foreign governments to attack military systems.

Brigadier General John Davis, the Army’s deputy commander for network operations, said that investment needed to be made into hardening the defenses of military systems rather than spending funds firefighting after the event.

"You can either pay me now or you can pay me later," said Davis.

"It would be nice to spend that money proactively ... rather than fixing things after the fact."