South Africa's gold producers said on Friday the National Union of Mineworkers (NUM) had served notice of a strike over pay demands starting from the night shift on Tuesday.

"The employers will continue to make plans in anticipation of strike action, to ensure the continuation of essential services, and to ensure the safety and security of employees and assets," the producers said in a statement.

"The employers remain open to discussions with all unions, and urge them to engage in good faith to reach an outcome that is acceptable to all," it said.

Affected companies include South Africa's four main gold producers, AngloGold Ashanti, Gold Fields, Harmony Gold, Sibanye, and some smaller operations.

Wage talks between the NUM, which represents about 64 percent of the over 120,000 workers in gold mining, and the companies broke down last week.

The country's Chamber of Mines, which negotiates on behalf of gold producers, said on Tuesday it had made a final offer to unions to increase basic wages by between 6 and 6.5 percent.

NUM has been seeking a 60 percent increase while a more hardline rival wants pay hikes of up to 150 percent. The companies say these demands are unrealistic, given rising costs and falling bullion prices.

A complete shutdown of the gold sector of indeterminate duration could cost South Africa more than $35 million a day in lost output, according to calculations based on the spot price.

This will put pressure on a struggling economy already weighed down by a ongoing strikes in auto manufacturing, construction and aviation services, and facing potential strikes from textile workers and petrol station employees.