Fritz Leutwiler, a Swiss banker appointed by South Africa to negotiate a rescheduling of the country's foreign debt repayments, said here today that President Pieter W. Botha had given him "clear indications" of intent to introduce significant political reforms soon.

At the same time, U.S. Assistant Secretary of State for Africa Chester A. Crocker, in the first visit by a senior member of the Reagan administration to one of South Africa's troubled black townships, learned today that a militant community leader he was scheduled to meet had been killed in an outbreak of racial violence.

Sources in the township of Leandra, east of here, said the local leader, Ampie Mayisa, 58, of the United Democratic Front movement, was stabbed to death in a clash last night with members of Zulu Chief Gatsha Buthelezi's Inkatha movement after his home was firebombed. Police confirmed the killing and said they had recovered the body.

Leutwiler, who has held three days of talks with government officials, said he believed that he had been given sufficient assurances of Pretoria's intentions on political change to enable him to persuade the creditor banks to accept a "silent understanding," if not a signed agreement, on an extended repayment of the $14 billion in frozen debts.

His impression contrasted sharply with that of a group of six U.S. congressmen, headed by House Budget Committee Chairman William H. Gray III (D-Pa.), who said at the end of a similar fact-finding tour Friday that they had found no willingness on the part of the Botha government to make substantial changes in the apartheid system of segregation.

The Pretoria government froze payments on its foreign debt last August when American banks, under pressure of growing antiapartheid sentiment in the United States, began a chain reaction of refusals to roll over South Africa's short-term loans.

Suddenly Pretoria found itself facing an impossible demand to repay $14 billion in short-term debts immediately. It declared a moratorium and called in Leutwiler, a former chairman of the Bank of International Settlements, to negotiate a rescheduling of payments.

After his initial meetings with the creditor banks in London last November, Leutwiler said he would be unable to persuade the banks to accept a rescheduling agreement unless South Africa introduced significant political reforms.

He was sharply critical of the government's failure to do this, and as racial unrest increased in the black townships, he told a Zurich newspaper in an interview that the situation in South Africa had deteriorated since he accepted the job of mediator.

But today, Leutwiler's attitude was transformed. After noting that South Africa's situation was "unique" in that it was not bankrupt but was facing demands for human rights reforms not made of other debtor countries, Leutwiler said that after talking with Botha and studying the speeches he had made during the past year he was convinced that there was "a very strong commitment" to reform.

"It is a hell of a program," Leutwiler added, saying that because of unobjective media reporting and the government's poor sense of public relations, the world was unaware of what South Africa was doing to end apartheid.

Leutwiler did not meet with any of the government's militant black opponents. The only blacks he met, he told a press conference today, were people who held office within the separatist administration, the most critical of whom was Buthelezi.

Asked why he had met no militants, the Swiss mediator replied that he had no interest in meeting people who received armed assistance from the Soviet Union.

"I don't like people who are in favor of violence, and I don't like people who want a civil war," Leutwiler added, in a clear reference to the outlawed African National Congress, the main black opposition movement that is trying to overthrow white-minority rule by guerrilla struggle.

Asked whether he thought American banks would be willing to sign a rescheduling agreement, Leutwiler said no, adding that he hoped his message of Pretoria's reformist intentions would be sufficient to achieve a "silent understanding" among all the creditor banks.

By this, Leutwiler seemed to be suggesting that while banks might be unwilling to sign a formal agreement, which would expose them to political criticism, he believed they would tacitly accept a deal to get their money back.

Meanwhile, the South African economy is showing signs of slight recovery, as the weakening dollar sent up the price of gold -- South Africa's main export -- to more than $340 per ounce on Friday, its highest level in nearly six months.

This caused South Africa's rand currency to recover to 42 U.S. cents, 8 cents higher than it was when Pretoria declared the debt freeze and imposed exchange controls to stop a flight of capital.

The modest improvements have caused an almost euphoric reaction here, with a progovernment Afrikaans-language newspaper declaring in a headline across its front page yesterday that a "glittering new year" lies ahead.

Crocker, in his visit to Duduza black township, called at the home of Joseph Thobela, who has been detained without charges under the emergency regulations since August.

Thobela's house has been gasoline bombed and two of his daughters have killed in the violence. Anglican Bishop Simeone Nkoane, who accompanied Crocker on his tour, said the assistant secretary talked for some time with Thobela's wife, Lefina.

"He seemed to be really concerned about the family. I think he saw something which he hadn't realized before," said Nkoane, who is an associate of Bishop Desmond Tutu, the 1984 Nobel Peace laureate.

Nkoane added that Crocker had been due to go on to the adjoining town of Leandra to meet Mayisa, but word was received that the local United Democratic Front leader had been killed. Crocker sought to avoid the press but did say "it was terrible that this should happen."