A US navy nuclear engineer and his wife have been charged with selling nuclear submarine secrets to what they thought was a foreign state.

Jonathan Toebbe, 42, and his wife Diana, 45, were arrested in West Virginia on Saturday, the Justice Department announced.

Toebbe worked in the US Navy's nuclear propulsion programme and had national security clearance.

The Justice Department said that in April 2020 Toebbe sent a package to a foreign government containing restricted data and a message suggesting a covert relationship, so that they could buy more data from him.

Also read:  US, UK equipping Australia with nuclear submarines as AUKUS alliance is born to counter China

The FBI intercepted the package, and the undercover agent posed as a representative of the foreign government in order to establish a relationship with the couple.

The agent sent Jonathan Toebbe, who used the pseudonym "Alice," an email offering him a gift as thanks for the data. But Toebbe responded with caution, asking instead to set up a "dead drop" location and to be paid in cryptocurrency.

For nearly a year, the couple "sold information known as restricted data concerning the design of nuclear powered warships to a person they believed was a representative of a foreign power," the statement said.

Jonathan Toebbe is reported to have raked in around $100,000 in cryptocurrency, the complaint affidavit alleges.

The couple transferred digital memory cards of data to the agent. In the first dead drop, "the SD card was wrapped in plastic and placed between two slices of bread on a half a peanut butter sandwich," the agent said in the complaint.

Other SD cards were hidden in a chewing gum package and a sealed band-aid wrapper, with the band-aid still inside.

The couple was arrested on Saturday after leaving another SD card at a dead drop location.