Apple says it will replace iPhone Duo handsets that have developed problems with their screens just days after the highly-anticipated smartphone went on sale.

Photos posted on social media show lines that appeared on the screens of the technology giant's first ever foldable device , which has a starting price of £1,999 ($2,668) in the UK.

Apple said it is aware of "an issue impacting the inner display on a small number" of the phones and urged affected users to contact the company for a replacement.

The Duo's launch, which attracted crowds at Apple stores around the world last week, comes as the brand tries to reinvigorate its range of devices and challenge the foldable phones made by rivals.

Some Duo owners also posted photos online showing cracks along the screen's centre crease, while others reported a crunching noise when opening the device.

An iPhone Duo user, Justin Lokos, posted an image of a device with lines across the right side of the display and later uploaded a picture of a new handset…