The US International Trade Commission has agreed to investigate Apple based on a complaint filed by handset maker HTC, the Commission said on Friday.
The agency is already investigating HTC after Apple filed its own complaint against the company. The dispute highlights how competitive the smartphone market has become.
HTC makes smartphones based on Google's Android software, as well as Windows Mobile devices. In May it asked the ITC to block the import of Apple iPhone, Apple iPad and Apple iPod products to the US, charging Apple with infringing five of its patents.
It hasn't stated publicly which patents it believes Apple infringed, but the ITC offered some hints. "The products at issue in this investigation are portable electronic devices that utilise certain power management methods and may incorporate hardware and software for telephone directories within mobile telephone systems," the agency said.
An administrative law judge for the ITC will hold an evidentiary hearing to look into HTC's complaint. The agency has 45 days to set a target date for completing the investigation.
The ITC has already agreed to investigate HTC based on a similar complaint filed earlier by Apple. Apple says that HTC uses patents related to the iPhone's user interface and a range of hardware and software technologies.
As the market for smartphones heats up, the main players are becoming increasingly litigious. Apple is also in a dispute with Nokia over patents. Research In Motion and Motorola settled a patent lawsuit on Friday.
Separately, HTC agreed to pay royalties to Microsoft for technology used in all of its phones running Android. Google, which designed Android and has an increasingly acrimonious relationship with Apple, has largely stayed out of the battle. After Apple filed its complaints against HTC, Google said it stood by the operating system and the partners that helped develop it.