Cape Town - The Democratic Alliance has called on President Thabo Mbeki to repudiate comments attributed to senior African National Congress members about national director of public prosecutions Bulelani Ngcuka's investigation of deputy president Jacob Zuma.

The Scorpions investigation unit, which falls under Ngcuka, is probing allegations of Zuma's involvement in the multibillion-rand arms deal.

DA justice spokesperson Sheila Camerer said in a statement on Friday that, in terms of the constitution, certain institutions were required to be independent.

When the DA objected to the appointment of ruling party members to these institutions, the African National Congress insisted that the appointees could, and would, be expected to act independently in the fulfilment of their duties.

"A senior ANC member who is quoted in this week's Mail & Guardian directly contradicts this claim," she said.

The newspaper quoted an unnamed senior ANC member as having said: "Bulelani knew the issue would damage the ANC and he should have consulted broadly within the ANC before starting.

"We all get deployed by the organisation and there are written and unwritten rules.

"One of the unwritten rules is to check with comrades before taking an action against the deputy president of the ANC," read the quote.

Camerer said Ngcuka had shown, through his prosecution of former ANC chief whip Tony Yengeni and former ANC Women's League president Winnie Madikizela-Mandela, and now his investigation into Zuma and former transport minister Mac Maharaj, that he was prepared to fulfil his original commitment on his appointment to be impartial and act independently.

"However, there is no doubt that he has been under immense pressure from within the ANC, and this is bound to weigh on his ability to do his job," she said.

"The national director of public prosecutions serves the people of South Africa.

"He is not there to serve the ruling party. Any suggestion that he is answerable first - or even at all - to the ANC should be condemned.

"The president needs to act to defend the independence and integrity of South Africa's independent institutions," said Camerer.

With acknowledgements to Sapa and News24.