Chinese government hackers compromised the computers of a US Navy contractor and stole a large amount of highly sensitive data on undersea warfare, including plans for a supersonic anti-ship missile for use on US submarines, the Washington Post reported on Friday, citing unnamed US officials.

The breaches took place in January and February, the officials told the Post, speaking on condition of anonymity about an ongoing investigation led by the Navy and assisted by the Federal Bureau of Investigation.

The FBI did not immediately respond to a Reuters request for comment.

"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," the US Navy said in response to a query from Reuters.

The Chinese Embassy knows nothing about the reported hacking, an embassy spokesperson told Reuters in an email, adding that the Chinese government "staunchly upholds cyber security, firmly opposes and combats all forms of cyber attacks in accordance with law."

The hackers targeted a contractor who works for the Naval Undersea Warfare Center, a military entity based in Newport, Rhode Island, the unnamed officials said without identifying the contractor, according to the Post.

The hacked 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 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.