Tens of thousands of staff at China’s main iPhone manufacturing base will have to go undergo mandatory Covid-testing on Thursday, a potential risk to global supply of Apple Inc.’s signature device.

Local authorities in Zhengzhou, where Foxconn Technology Group operates its main iPhone-making complex, ordered compulsory testing for areas that include the plant and adjacent parts of the city. Foxconn representatives didn’t have immediate comment.

The mass-testing exercise represents a risk because Chinese officials have been quick to lock down areas in the event of positive tests. This week, more than 30 Taiwanese companies including iPhone assembler Pegatron Corp. and Apple laptop maker Quanta Computer Inc. have now halted production in the electronics hubs of eastern China to comply with local Covid-related restrictions, spelling more trouble for an already fragile global tech supply chain.

President Xi Jinping has championed a zero-tolerance approach to Covid that includes rapid lockdowns if even a small number of people test positive. His administration has held firm, even as public anger simmers in Shanghai and economic costs mount.

“Prevention and control work cannot be relaxed,” Xi said during a trip to the island province of Hainan, the official Xinhua News Agency reported late Wednesday, the same day Shanghai saw a record 27,719 new cases.