South African Deputy President Cyril Ramaphosa yesterday called on prosecutors to act urgently in pursuit of a firm owned by friends of scandal-plagued President Jacob Zuma, as pressure mounted on him to step down.

Ramaphosa succeeded Zuma as head of the ruling African National Congress last month, making him likely to replace Zuma as the country's next president in 2019 - or even earlier, if Zuma resigns.

In recent days he has gone on the offensive against companies controlled by the Gupta family, businessmen friends of Zuma accused of unduly using political connections to win work with the state.

"We want to deal with the rot. It is unacceptable, totally unacceptable, that companies owned by the nation and set up to benefit the people have been hijacked," Ramaphosa said at a meeting of business leaders and cabinet ministers.

The rand currency has rallied around 16 percent since Ramaphosa's ANC leadership victory as investors bet he will crack down on corruption and implement policy reforms.

The Gupta family and Zuma have denied wrongdoing and say they are victims of a politically-motivated witchhunt. A spokesman for Zuma and a lawyer for the Gupta family did not respond to requests for comment yesterday.

South Africa's state prosecutor said on Wednesday it would serve a court order on McKinsey relating to a 1.6 billion-rand ($130 million) contract with state utility Eskom that the global consultancy worked on with Trillian, a local firm that was then controlled by Gupta family associates.

"We welcome the actions they are beginning to take and we encourage them to act with urgency," Ramaphosa said.