President Thabo Mbeki joined others on Tuesday in calling for a speedy conclusion to the Scorpions' probe into Deputy President Jacob Zuma's alleged involvement in the multi-billion rand arms deal.

"You can't have it dragging on for ever. The sooner the matter is concluded, the better," Mbeki told reporters in Pretoria.

He said he had also conveyed this sentiment to Justice Minister Penuell Maduna, who is responsible for the corruption busting unit.

Mbeki expressed concern about the allegations and counter-allegations on the issue, reported in the media over the past few days.

Zuma has condemned the leaking to the Sunday Times newspaper of 35 questions Scorpions detectives had asked him to answer on his alleged misconduct.

This was "totally unacceptable and despicable", the deputy president said.

"Despite my silence ... there have been constant leaks of information to the media, which was clearly designed to cast aspersions on my integrity."

National Director of Public Prosecutions, Bulelani Ngcuka, under whose office the Scorpions unit operates, has denied the questions were leaked by the unit, as has Maduna.

Ngcuka has himself said he is the target of a political campaign because of the investigation.

On Tuesday, Mbeki said he doubted the debate over the leaks helped the investigators or those being probed.

The mere fact of the allegation against Zuma was a matter of concern.

"But I suppose the prosecuting authorities would not have said what they have said or done what they are doing if they didn't think they had just cause to do it," Mbeki said.

"And I would imagine the deputy president wouldn't make the statements he is making unless he was convinced that he has just cause to say the things he is saying.

"I really do wish that this matter could move forward as speedily as possible so that the thing could get resolved, whichever way," Mbeki said.

As the Democratic Alliance called for Zuma to step down on Tuesday, pending the investigation, the SA Communist Party also urged the probe be completed as soon as possible, and the New National Party warned against attacking the Scorpions' credibility.

Raenette Taljaard of the DA said Mbeki should explain why he was not applying the "Maharaj private sector principle" to Zuma.

He should also explain why his office sought to protect the deputy president as far back as September 1999, when it stated, among other things, that "the Presidency rejects any insinuation that Deputy President Zuma is implicated in shady arms deals", she said.

NNP spokeswoman Carol Johnson said the freedom and integrity of the press was a cornerstone of a proper democracy, and to expect it to reveal its sources was shocking.

Johnson added that "to attack the credibility of the Scorpions is, at this point, unjustified and based on assumption".

"Until such time as there is actual proof that the leak came from the side of the Scorpions, they should not be made a scapegoat," she said.

The SACP condemned the leak, and said as much as the media might have an important investigative role, this should be balanced with the need for an effective and fair investigation process.

No matter who leaked the questions, it was tantamount to "trial by the media".

The SACP believed the focus should be on bringing the whole matter to speedy finalisation.

The sooner any allegations against Zuma were finalised through proper legal processes, and not the media, the better, it said.

Zuma reportedly reiterated on Tuesday he would co-operate fully with the Scorpions.

However, his spokeswoman Lakela Kaunda was not available for comment, and had not responded to messages by late afternoon.

With acknowledgment to Sapa.