The mines produce coal for Mittals huge steel plant in the region in central Kazakhstan but the company has said it has plenty of reserves to avoid production problems.
Another two mines from the Karaganda basin have joined the four that were already on strike, coal trades union leader Vyacheslav Sidorov told Reuters by telephone.
The only people going into the mine shafts are those who need to keep the life support systems running. No coal is being mined, he said, adding the union was in talks with the firm.
Sidorov could not give an exact number of strikers. The eight mines employ 24,500 people.
There was no immediate comment from Mittal Steel Temirtau, the Mittal Steel subsidiary in Kazakhstan, on Thursday.
The striking miners are demanding new equipment and want wages raised to $15 an hour or to about $2,500 a month due to dangerous working conditions. They are paid $430 compared to the average wage in Kazakhstan of $320 a month.