TAIPEI—Foxconn Technology Group has resumed hiring assembly-line workers in China after a postholiday freeze, in the latest sign that customer Apple Inc. is gearing up for production of a new iPhone.

Foxconn is the trade name for Taiwan-based Hon Hai Precision Industry Co. The Taiwanese company said Monday it has added about 10,000 assembly-line workers a week in Zhengzhou, its major production facility for iPhones, since the last week of March.

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