US network AT&T hinted on the impending arrival of iPhone 6 and/or iPhone 5S on Monday at Apple's WorldWide Developers' Conference 2013.

The network now offers early upgrade and trade-in offers, informing several customers that their upgrade eligibility dates have been moved up and encouraging them to take the plunge on an iPhone 5 now.

The carrier usually employs such strategy when the launch of the next generation iPhone nears. This is to dispose all of the remaining stocks of the previous model and make way for the new one.

AT&T customers said that the carrier has reached out to them with the offer, which includes the option to trade in an older iPhone in exchange for a free iPhone 5.

Moreover, this year's WWDC logo hinted the unveiling of the smartphone in different colors. The WWDC logo was unveiled to be a multi-color collage of red, blue and green just like that of the iPod Touch. It has long been rumoured that the next iPhone will be available in different color variants.

The unannounced iPhone 6 and iPhone 5S are clouded by a myriad of rumours. A lot of concept renders are emerging on the internet as the devices' release date nears.

Rumour has it that the next iPhone will feature Retina or IGZO screen display, A7 quad core chip processor, iOS 7, 128GB internal storage, 13 mega-pixel camera, and longer battery life. The lack of NFC support on the iPhone 5 was one of the biggest criticisms aimed at Apple's smartphone.

Meanwhile, the cheaper iPhone 5S is rumoured to sport a plastic casing, 4-inch screen display, dual core processor and 8-megapixel camera.

It is initially reported that the new iPhones will be launched in June along with the upgraded iOS. However, other reports stated that the devices will make its debut in September.

The Apple chief Tim Cook also dropped several hints that the upcoming iPhone 6 and 5S will be released later this year. Cook said that no new products are to launch before the end of the year.