The Pentagon spent more than $100 million in the past six months to repair damage from cyber attacks and other computer network problems, military leaders said Tuesday.

It is just beginning to track the costs, which are triggered by constant daily attacks against military networks ranging from the Pentagon to bases around the country, this is indeed our big challenge as we think about how to defend it, said Kevin Chilton, Air Force General.

The money was spent on manpower , computer technology and contractors hired to clean up after both external probes and internal mistakes, according to Army Brig. Gen. John Davis, deputy commander for network operations.

Officials want to find out how much of the $100 million cost was due to outside attacks against the system, versus viruses and other problems triggered accidentally by Defense Department employees. They decided to reveal details about suspected cyber attacks against the e Pentagon by other countries, such as China.

The Pentagon has acknowledged that its vast computer network is scanned or probed by outsiders millions of times each day.

Last year a cyber attack forced the Defense Department to take up to 1,500 computers off line. And last fall it also banned the use of external computer flash drives because of a virus threat officials detected on the Pentagon networks.