SAN FRANCISCO — Reports of a small number of batteries in Apple's new iPhone 8 Plus phones swelling and causing the case to pop open have been circulating since just after they went on sale last month. On Friday Apple said it was "looking into the matter."

The phones don't appear to overheat or catch fire, as the Samsung Galaxy Note 7 did. The battery simply swells, causing the phone's casing to open and the front screen to detach.

In all the reported instances, the battery inside the phone swelled, causing the phone's casing to bend and then pop open and the screen to detach and rendering the phone nonfunctional.

MacRumors, one of the first news outlets to report on the battery swelling problem, has found five instances out of the millions of phones that have been sold around the world.

The earliest report appears to have come on September 26 when a woman in Taiwan charged a new iPhone 8 Plus and noticed that the battery had swollen and the case was cracked open.

The iPhone 8 went on sale on Sept. 22.

iPhone 8 lines were orderly and short. In other words, a snooze

On Thursday the Chinese news site, thepaper.cn, carried a story about a Guangzhou resident whose new iPhone 8 Plus had already burst before he opened the box the day it was delivered. According to the news site, neither the company he bought the phone from nor Apple would take responsibility for replacing it.

Several iPhone 8 Plus buyers tweeted out photos of the problem. In Japan, Magokoro0511 reported that the phone was swollen when he first opened the box.