The strike, now in its third week, has curbed exports of metals, cars, fruit and wine to Europe and Asia, as well as imports of vehicle parts and fuel supplies three weeks before the soccer World Cup starts.

"Our members have returned to work," said George Strauss, President of the United Transport and Allied Trade Union (Utatu).

The smaller union at Transnet, the South African Transport and Allied Workers Union (Satawu), said its members remained fully behind the action.

"The strike is still on, none of our workers are back to work," said Zenzo Mahlangu, General Secretary at Satawu, which represents 39 percent of the 54,000 workers at Transnet.

Transnet said no meetings were planned with Satawu for Monday, but the rejected pay offer was still on the table.

Economists have estimated losses in the hundreds of millions of rand, but this could rise to billions if the strike drags on, and it may take weeks to clear the backlog at ports.

South Africa hosts the World Cup in June and July, and soccer's world governing body FIFA said imports of some equipment for the event have been affected.

So far, coal exports to power plants in Europe and Asia have not been affected thanks to stocks at the ports, and fuel supplies to petrol pumps are also as yet unaffected.

The strikers are under pressure from the government to end the stoppage, while the workers themselves are feeling the pinch because they are not being paid while off the job.

More protests loom before the tournament.

A strike by half of the workers at state-owned power utility Eskom due to start on Wednesday could disrupt electricity supplies in Africa's biggest economy, and embarrass President Jacob Zuma's government before one of the world's premier sports events.

Civil servants, including nurses, police and teachers, may also consider striking unless a mediator helps to resolve a wage dispute with the government.

The transport strike has hurt global miners with operations in South Africa, forcing some to declare force majeure, and car makers — a big employer and contributor to the economy — warned that they would shut their operations if it persists.

Anglo American Plc, Xstrata and the world's top steelmaker ArcelorMittal declared force majeure on the supply of iron ore, ferrochrome and steel respectively. Transnet also declared force majeure on coal destined for export.