Electronic eavesdropping is the National Security Agency's forte, but it seems it also has a special interest in children's car seats, Foreign Policy magazine reported Wednesday.

The "integrated child seat for vehicle" is among more than 270 patents issued since 1979 to the super-secretive spy agency at the center of whistleblower Edward Snowden's revelations.

Patented in 1993, the device looks something like a precursor to built-in, fold-out rear child seats available today in some Chrysler and Volvo automobiles.

"The national security benefits of this device are neither obvious nor spelled out in the patent," wrote Foreign Policy on its Complex blog (complex.foreignpolicy.com).

"But the car seat's inventors promise to finally overcome a 'well known measure of inconvenience' plaguing parents across America, who are forced to install new, bigger car seats as their children grow up, the patent states."

Other NSA patents come across as more obvious, including data encryption methods, voice-recording analysis and ways to strip distortion out of intercepted communications.

There's also a patent for a device that would let spies know if a SIM card in a mobile phone has been removed or replaced.

In an email to AFP, the agency's public affairs office acknowledged: "It is NSA's practice to seek patents for inventions."

It added that, once a patent is issued, the NSA can license it to others -- so long as doing so is deemed to be "in the agency's best interest."