A report in the United States says Chinese hackers have gained access to the designs of more than two dozen major US weapons systems.
The Washington Post, citing a Defence Department report, says compromised weapons designs include those for combat aircraft and ships, as well as missile defences vital for Europe, Asia and the Gulf.
Among the weapons listed in the report were the advanced Patriot missile system, the Navy's Aegis ballistic missile defense systems, the F/A-18 fighter jet, the V-22 Osprey, the Black Hawk helicopter and the F-35 Joint Strike Fighter.
The report did not specify the extent or time of the cyber-thefts or indicate if they involved computer networks of the U.S. government, contractors or subcontractors.
But the espionage would give China knowledge that could be exploited in a conflict, such as the ability to knock out communications and corrupting data, the Post said. It also could speed China's development of its defense technology.
In a report to Congress this month, the Pentagon said China was using espionage to modernize its military and its hacking was a serious concern, Reuters reports.
The Pentagon report said the US government had been the target of hacking that appeared to be "attributable directly to the Chinese government and military."
China dismissed the report as groundless. China also dismissed as without foundation a February report by the U.S. computer security company Mandiant, which said a secretive Chinese military unit was probably behind a series of hacking attacks targeting the United States that had stolen data from 100 companies.
Earlier, Australian media said Chinese hackers have stolen the blueprints for Australia's new spy headquarters in Canberra, a claim denied by China.
An ABC news report said hackers linked to China stole the floor plans of a A$630 million headquarters for the Australia Security Intelligence Organisation, the country's domestic spy agency.
The attack through the computers of a construction contractor exposed not only building layouts, but also the location of communication and computer networks, it said.
The Chinese government has denied Chinese hackers stole blueprints. Government spokesperson Hong Lei said China attaches importance to cyber security and opposes hacking in any form.