By Ed Cropley

JOHANNESBURG (Reuters) - South African rail freight operator Transnet is bracing for a one-day strike by port and rail workers in support of a walkout by 20,000 truckers that has hit fuel supplies round the economic hub of Johannesburg and at least one car plant.

Large parts of South Africa's gold and platinum mining sectors have been brought to a standstill in the last two months by a wave of wildcat labor unrest in which almost 50 people have been killed -- 34 of them by police.

State-owned Transnet said on Sunday it had been served with a notice of a walkout "in a week's time" by the SATAWU transport union behind the two-week trucker stoppage, but did not say when it might occur.

"We are considering the notice and will activate our contingency measures to ensure minimal disruptions should the action materialize," Transnet said in a statement.

SATAWU spokesman Vincent Masoga said wage talks between freight bosses and unions were scheduled to restart on Tuesday after breaking down acrimoniously at the end of last week, but the union was still gearing up for action.

"We've issued notices. It is going ahead. We are mobilizing," Masoga said.

Top producer Anglo American Platinum fired 12,000 illegal strikers on Friday, raising fears of even more violence around the "platinum belt" city of Rustenburg although the weekend passed off largely without incident.

The mine strikes look set to knock already shaky economic growth in Africa's biggest economy and have already triggered a sharp sell-off in the rand. The trucker strike, if it persists, could have a far harsher and wider effect.

Oil giant Shell said on Friday it could not honor fuel delivery contracts around Johannesburg, declaring "force majeure" to free itself and customers from existing obligations, and other petrol suppliers are holding their breath.

General Motors reported disrupted production at its plant in the southern city of Port Elizabeth.

President Jacob Zuma and his African National Congress have been criticized as slow to respond to the strikes, although the ANC's long-standing ties to the unions and a looming internal leadership election mean decisive action is unlikely.

Zuma is favored to win re-election at the December conference, teeing him up for a second five-year term as head of state in 2014, although domestic media are alive with rumors he could face a challenge from Deputy President Kgalema Motlanthe.

The speculation moved up a gear on Sunday with newspapers publishing excerpts of a new Motlanthe political biography, even though in the book Motlanthe refused to be drawn on whether he would accept a nomination to run against Zuma.

(Reporting by Ed Cropley; editing by Ron Askew)