Asustek Computer has seen consumers in the China market have returned nearly 200,000 ZenFone smartphones due to issues when upgrading from Android 4.4 to Android 5.0, which is expected to affect sales of ZenFone 2 in China and consequently pose a challenge to Asustek's target global shipments of 25 million smartphones for 2015, according to industry sources.

Asustek shipped three million ZenFones in the first quarter of 2015 and expects to ship 4.8 million units in the second, the sources said. The 2015 target shipments of 25 million ZenFones includes target shipments of eight million units in China, the sources indicated.

Asustek expects to ship nearly two million ZenFones in June and aims to ship three million a month, mostly ZenFone 2 units, in the third quarter, the sources said.