TUMAHOLE, SOUTH AFRICA, FEB. 14 -- The trial of Winnie Mandela on kidnapping and assault charges was postponed today until early March to give police time to locate a missing prosecution witness amid growing indications that the state's case against the wife of African National Congress leader Nelson Mandela is collapsing.

Supreme Court Judge M. S. Stegmann also ruled that two other state witnesses did not have "a just excuse" for refusing to testify out of fear of reprisals and ordered them taken into custody. But the judge later postponed their arrest until March 6, when the trial is scheduled to resume.

State prosecutor Jan Swanepoel, who had asked for the postponement, said he hoped that the delay would enable police to find Gabriel Mekgwe, a key witness who disappeared from a Methodist Church home in Soweto the night before the trial began Feb. 4.

The other two witnesses, Kenneth Kgase and Barend Mono, told the court Wednesday that they would not testify against Winnie Mandela because they feared for their lives.

Swanepoel said today that if Mekgwe were found unharmed and agreed to testify, he believed the other two witnesses would change their minds and give testimony when the trial resumes. If Mekgwe is still missing by then, the state would have to decide how and whether to proceed, Swanepoel told the court.

Earlier, Swanepoel told reporters he thought it would be "a waste of time" to continue the trial without the testimony of these three surviving victims of an alleged kidnapping and assault, for which Mandela and three co-defendants are on trial.

Here in this Orange Free State township, meanwhile, the mother of "Stompie" Moketsi Seipei, 14, who was killed after the alleged kidnapping, said she is still waiting for Mandela, "the mother of the nation . . . just to come here and clear up these rumors" about her alleged role in his death.

The woman, Mananki Seipei, spoke wtih reporters at a press conference called by local ANC officials who had complained about a recent newspaper article that said Seipei was being kept from speaking freely.

But three local ANC leaders sought to shield Seipei from direct questioning, answering many questions for her. After reporters insisted on hearing from her directly, she finally said that while she was "not angry" at Mandela, she also was "not satisfied" and wanted to hear Mandela's version of what had happened.

Stompie Seipei was allegedly kidnapped and taken to the Mandela home in Soweto the night of Dec. 29, 1988, together with the prosecution's three witnesses. The three already testified in an earlier trial that they were severely beaten in her presence and with her participation. Seipei was found dead in a field three days later, and Mandela's bodyguard, Jerry Richardson, was convicted of murdering him.

Mandela was not charged in his death but with involvement in the alleged kidnapping and assaulting of the four youths.

Today's session of the Mandela trial in Johannesburg was taken up mostly with Judge Stegmann's hour-long summary of arguments for and against the refusal of Kgase and Mono to testify. Their fears of future reprisals, he said, were "too remote and speculative" to justify excusing them from testifying. He concluded that "neither witness has proved that they have a just excuse."

He first ordered them to be taken immediately into custody but then gave them 25 minutes to reconsider while the court adjourned. When it resumed, the two again refused to testify.

But Swanepoel asked the judge to postpone their sentencing until early March. He also asked that they not be held in protective custody against their wishes and said the state would provide protection if requested.

Asked later if he were frustrated by the loss of his three main witnesses, Swanepoel replied: "I think everyone is frustrated here . . . but there is nothing we can do about it."