China's state hackers have reportedly stolen a large amount of highly-sensitive US navy data on its undersea warfare, including plans for supersonic anti-ship missiles on submarines.

Government experts were said to have compromised the computers of a US navy contractor, giving them access to the information, according to the Washington Post, citing unnamed American officials.

They said - on the condition of anonymity about an ongoing investigation - that the security breaches were believed to have taken place in January and February.

Hackers reportedly targeted a contractor which works for the Naval Undersea Warfare Center - a research and support centre in Newport, Rhode Island.

The officials did not identify the contractor.

It was reported that a total of 614 gigabytes of data was taken. The information related to a secret project known as Sea Dragon.

Also accessed were 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 said it agreed to withhold some details at the request of the US navy over fears it could compromise national security.