It's been revealed that some airlines have seat-back entertainment systems that contain cameras.

American Airlines, United Airlines and Singapore Airlines all confirmed that they had cameras on their newer entertainment systems however said that they had never activated them.

It's likely that other planes used by other carriers may also contain the cameras.

The issue was raised after a passenger on a Singapore Airlines flight posted a photo of the seat-back display last week and the tweet was shared several hundred times.

Buzzfeed first reported that the cameras are also on some American planes.

American Airlines confirmed the cameras and said that companies that make the entertainment systems have installed them to offer future options such as seat-to-seat-video conferencing.

All three airlines stressed that they didn't add the cameras - manufacturers embedded them in the entertainment systems.

American's systems are made by Panasonic, while Singapore uses Panasonic and Thales, according to airline representatives. Neither Panasonic nor Thales responded immediately for comment.

As they shrink, cameras are being built into more devices, including laptops and smartphones. The presence of cameras in aircraft entertainment systems was known in aviation circles at least two years ago, although not among the travelling public.

Seth Miller, a journalist who wrote about the issue in 2017, thinks that equipment makers didn't consider the privacy implications.

"There were already cameras on planes - although not so intrusive - and the companies assumed that passengers would trade their images for convenience, as they do with facial-recognition technology at immigration checkpoints, he said.

"Now they're facing blowback from a small but vocal group questioning the value of the system that isn't even active," Miller said.

American Airlines spokesman Ross Feinstein said cameras are in "premium economy" seats on 82 Boeing 777 and Airbus A330-200 jets. American has nearly 1,000 planes.

"Cameras are a standard feature on many in-flight entertainment systems used by multiple airlines," he said.

Singapore spokesman James Boyd said cameras are on 84 Airbus A350s, Airbus A380s and Boeing 777s and 787s. The carrier has 117 planes.

While the airlines say they have no plans to use the cameras, a Twitter user named Vitaly Kamluk, who snapped the photo of the camera on his Singapore flight, suggested that just to be sure the carriers should slap stickers over the lenses.

"The cameras are probably not used now," he tweeted. "But if they are wired, operational, bundled with mic, it's a matter of one smart hack to use them on 84+ aircrafts and spy on passengers."