The breaches took place in January and February, US officials told the Washington Post, speaking on condition of anonymity about an ongoing investigation led by the Navy and assisted by the Federal Bureau of Investigation.
The US Navy declined to comment other than to say: "Per federal regulations, there are measures in place that require companies to notify the government when a 'cyber incident' has occurred that has actual or potential adverse effects on their networks that contain controlled unclassified information. It would be inappropriate to discuss further details at this time."
A spokesman for the Chinese Embassy in Washington said officials knew nothing about the reported hacking, according to Reuters, adding that the Chinese government "staunchly upholds cyber security, firmly opposes and combats all forms of cyber-attacks in accordance with law".
The hackers are believed to have targeted a contractor working for the Naval Undersea Warfare Centre, on Rhode Island.
The material comprised 614 gigabytes relating to a project known as Sea Dragon, as well as signals and sensor data, submarine radio room information relating to cryptographic systems and the Navy submarine development unit’s electronic warfare library, the Washington Post reported.
The newspaper said it had agreed to withhold some details about the compromised missile project after the Navy said their release could harm national security.
The data stolen was of a highly sensitive nature despite being housed on the contractor’s unclassified network, the Post said, citing the officials.