In a comment to the Post, however, a Navy spokesman said there's a process in place for third parties to notify the government when a breach may have affected controlled unclassified information, but that it would be "inappropriate to discuss further details".
For the importance of the materials and the potential damage their loss can cause, investigators say it took the intruders surprisingly little effort to pilfer them, since the network that stored the data was unclassified.
The hacks took place in January and February and targeted an unidentified contractor who works for the Naval Undersea Warfare Center in Newport, R.I., that conducts research and development for submarines and underwater weaponry, The Washington Post reported, citing USA officials.
The hackers stole "massive amounts" of secret data from the Navy contractor - including plans to develop a supersonic anti-ship missile for USA subs, American intelligence officials told the Post.
Hackers targeted a contractor linked to a USA military organisation that conducts research and development for submarines and underwater weaponry.
The Navy and FBI are investigating the breach. Bill Speaks was quoted as telling the Post.
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The Post "agreed to withhold certain details about the compromised missile project at the request of the Navy, which argued that their release could harm national security".
The data that was compromised by what officials believe were Chinese hackers working for the state intelligence agency concerns the Sea Dragon project, which is aimed at "integrating an existing weapon system with an existing Navy platform". The sanctions were imposed because ZTE breached a USA embargo on trade with Iran and other countries for six years and lied about it.
The revelation of the cyberattack, which happened last winter, comes days before President Trump attends a summit in Singapore with North Korean leader Kim Jong Un, who counts Beijing among his allies.
The Pentagon has spent more than $300 million since 2015 on the Sea Dragon project, which involves adapting current Navy technology for new uses.
Military experts said the breach signaled that China could be working to complicate the Navy's ability to defend USA allies in Asia amid a conflict with Beijing in the region.