Chinese government hackers broke a US naval contractor and made off with more than 600 gigabytes of data.

The breaches occurred in January and February and affected a contractor working on submarines and underwater weapons.

The Post said that those behind the breach are alleged to have obtained secret plans for a supersonic anti-ship missile.

Report says the data was housed on the contractor’s unclassified network, when aggregated, officials said the data could be considered classified.

According to the New York Times, the data taken was unclassified.

Both media outlets cited unnamed officials and the contractor targeted in the cyber-attack was not identified by either.

“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.

The Chinese Embassy told Reuters that it knew nothing about the reported hacking, adding that the Chinese government “staunchly upholds cyber security, firmly opposes and combats all forms of cyber-attacks in accordance with law”.