Industrial action in South Africa's oil sector that is threatening refineries and fuel supplies intensified on Wednesday after a small but influential union said it would join other labour groups already on strike.

Tens of thousands of workers in the petroleum, chemical and pharmaceutical industries stopped working from Monday, demanding 13-per-cent wage increases, about triple the inflation rate, and above the 4 to 7 per cent offered by employers.

The smaller Solidarity union, which represents about 6,000 skilled workers across the sector, plans to soon join about 70,000 striking workers, mainly from the Chemical, Energy, Paper, Printing, Wood and Allied Workers Union.

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"The strike most definitely will continue next week," said John Appolis, CEPPWAWU's national policy co-ordinator, adding there were no planned talks with employers.

Solidarity said its "members in this sector, specifically at PetroSA, will strike from tomorrow. Employees at Sasol Secunda are also expected to join the strike soon."

Africa's biggest economy has already started to feel the fuel pinch, with shortages recorded in Gauteng province, the country's economic hub and home to its biggest city, Johannesburg.

"So far, 16 service stations in Johannesburg have gone dry, as well as two in Pretoria and one in Durban," said Tania Landsberg, spokeswoman at fuel retailer Engen, which runs 114 service stations in Gauteng and 1,200 nationally.

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Logistics firm GAC said that berthing and bunker-fuel delivery delays to ships at Cape Town were expected.

Operations at South Africa's largest, 180,000 barrels-per-day Sapref refinery, jointly owned by BP PLC and Royal Dutch Shell, were still continuing.

"Sapref has sufficient numbers of competent personnel who are outside the bargaining unit to continue operating safely," said spokeswoman Margaret Rowe, but declined to say if the refinery was still operating at full capacity.

Other employers in the sector include petrochemicals group Sasol, state-owned energy group PetroSA, Chevron and Total.

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"This situation is not sustainable. The backlog will catch up slowly and affect production. The reports of service stations going dry mean that in effect the product is sitting at depots and going nowhere," said Avhapfani Tshifularo, executive director of the South African Petroleum Industry Association.

Unions and employers in Africa's biggest economy are in their mid-year bargaining session known as "strike season," with many labour groups seeking wage increases that far exceed inflation.

Central bank and Treasury officials have said high wage increases threaten the outlook for inflation and the long-term prospects for the economy.

A strike in the steel and engineering sector entered its second week on Monday, with the industry expecting substantial production and financial losses. Solidarity union said it had rejected an offer proposed by the bargaining council.

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Wage talks in South Africa's mining sector are also well under way, and possible strikes loom in the crucial platinum, coal and gold industries.