South Africa's President Jacob Zuma answers questions at Parliament in Cape Town Thomson Reuters JOHANNESBURG (Reuters) - Pressure on South Africa's President Jacob Zuma intensified on Sunday when in a newspaper interview, a former high-ranking official added to mounting allegations that Zuma had a corrupt relationship with a prominent family of business owners.

Zuma has so far insisted his ties with the Gupta family are above board, but investors fear further political uncertainty could hasten a credit ratings downgrade, potentially into "junk" territory, and sharply raise South Africa's borrowing costs.

The allegations also reinforce concerns over governance and stability in Africa's most industrialized economy. South Africa's opposition has called on Zuma to resign over the row.

Former cabinet spokesman Themba Maseko told the Sunday Times newspaper that in 2010 Zuma asked him in a telephone call to meet the Gupta family at their home in Johannesburg and to "please help them."

Maseko said he met two Gupta brothers who wanted his help in directing government advertising to a newspaper that the wealthy family was launching, the report said. Gupta family spokesman Gary Naidoo rejected Maseko's accusations, it said.

The presidency was not immediately available to comment.

In the past week, several senior officials have accused the Guptas of wielding undue influence in government activities. The family of Indian-born businessmen who moved to South Africa in the early 1990s, says it is the victim of a plot to oust Zuma.

Zuma's son, Duduzane, is a director along with Gupta family members of at least six companies, documents show.

Deputy Finance Minister Mcebisi Jonas said on Wednesday the Gupta family had offered him former finance minister Nhlanhla Nene's job shortly before Zuma abruptly dismissed Nene in December.

Zuma has said only he appoints ministers to the cabinet and dismissed Jonas' account, and the Guptas have also denied trying to influence political appointments.

Nene's sacking sent South Africa's rand currency down nearly 10 percent in December. Zuma gave no reason for sacking Nene, who was keen to rein in government spending, shocking markets.

Zuma's African National Congress (ANC) is holding a three-day meeting of the party's leadership that concludes on Sunday. The party faces stiff competition from the opposition at municipal elections this year including the commercial hub of Johannesburg and capital Pretoria.

The party has said there were no plans to address the issues surrounding Zuma's ties with the Guptas. Zuma has survived several scandals over the years.

(Writing by James Macharia; Editing by Raissa Kasolowsky)