Cyber attacks on America will continue to escalate, according to National Security Director Keith Alexander, speaking to the Reuters Cybersecurity Summit in Washington.

“Disruptive and destructive attacks on our country will get worse," said Alexander, the leading U.S. general in charge of the nation’s cybersecurity.  "Mark my words, it will get worse."

Speaking at the Reuters Cybersecurity Summit in Washington last week, Alexander described cyber espionage as “the greatest transfer of wealth in history.”

U.S. Secretary of Homeland Security Janet Napolitano said at the same summit that her main concern was with “the known unknown”.

Napolitano said that the recent heist which used ATM machines around the world to withdraw $45 million via prepaid debit cards offered an illustration of the scope of cybercrime.The attack targeted prepaid credit cards. By raising the limit on cloned cards the hackers were able to withdraw “unlimited” funds for short periods. In New York, the hackers withdrew $2.8 million in hours.

“We don't have the identity of all the adversaries who are trying to either commit crimes or acts over the cyber networks,” said Napolitano. “The things we know about, we can deal with. It's the known unknown.”