This latest incident followed tighter rules to prevent tiny indentations on the phones and scratches to the phones' frames and back covers. The new iPhone 5 is said to be more susceptible to such markings.

A fight between workers and quality-control inspectors resulted in some injuries and people being taken to the hospital, China Labor Watch said.

"They have such high expectations for these products, even if you raise the demands a little bit it makes a huge difference to the pressure on the workers," Li Qiang, China Labor Watch's executive director, said in an interview.

He last heard from workers at the plant late Friday night and the strike had not been resolved, he said.