Chinese hackers have accessed designs for more than two dozen US weapons systems, a US newspaper has reported.
Designs for combat aircraft, ships and missile defences were among those compromised, a Pentagon paper found, the Washington Post reported.
A public version of the same Defense Science Board report said in January that the US was unprepared for a full-scale cyber attack.
A Pentagon spokesman said "intrusions" had not eroded its technological edge.
The Defense Science Board did not return requests for comment from the BBC.
The Washington Post report comes as Australia discloses Chinese hackers stole floor plans for the new headquarters of its domestic intelligence agency.
The compromised US designs include those for advanced Patriot missile systems called PAC-3, an Army anti-missile system known as Thaad, and the Navy's Aegis ballistic-missile defence system, according to the Washington Post.
The F/A-18 fighter jet, V-22 Osprey aircraft, Black Hawk helicopter and the Navy's new Littoral Combat Ship were also compromised.
The F-35 Joint Strike Fighter, the most expensive weapons system ever built, was also cited on the list.
The reports do not describe the extent of the theft, but correspondents say the hack could give China information that could be used against the US in the event of a conflict.
In a statement, US defence department spokesman George Little said the Pentagon maintained "full confidence in our weapons platforms".
"The Department of Defense takes the threat of cyber espionage and cyber security very seriously, which is why we have taken a number of steps to increase funding to strengthen our capabilities," he said.
"Suggestions that cyber intrusions have somehow led to the erosion of our capabilities or technological edge are incorrect."