Apple has quietly launched a new smart battery case for the iPhone 6s and iPhone 6 that addresses users' concerns about the battery life of its smartphones.

The case can increase talk time to 25 hours, and support Internet use on LTE for up to 18 hours, Apple said on its website. The battery is charged via a standard Lightning port.

This is the first integrated battery pack for the iPhone released by Apple after facing years of complains over the smartphone's poor battery performance.

The case comes in two colours, white and charcoal grey, and is priced at $148 on the Singapore online Apple store.

Not everyone was wowed by the new battery case, which slips around the iPhone like a regular case. Some Apple users took issue with the shape, in particular the hump on the back.

But reviewers noted that while some battery cases made by third parties had sleeker designs, the Apple-issued case is integrated with iOS - the status of the case's battery is shown on the iPhone's screen, along with the status of the phone's battery.