Six U.S. congressmen, led by Pennsylvania Democrat William H. Gray III and including nonvoting D.C. delegate Walter E. Fauntroy, began an intensive tour of South Africa today to assess the effects of limited American sanctions on the country's apartheid system of segregation.

"We must see if the sanctions are working or if they should be toughened," said Peter H. Kostmayer (D-Pa.) when the group landed at Johannesburg airport last night. "We are also here to express our solidarity with the [black] majority in South Africa and our sympathy with their aspirations," he added.

The group suffered an early rebuff when their request to meet imprisoned black nationalist Nelson Mandela was refused. Gray indicated that he would renew the request when the group meets President Pieter W. Botha Wednesday.

The group's visit coincides with an apparent freezing of the Pretoria government's program of limited change and a return to hawkish regional policies. This comes in the face of increased international pressures and an intensification of insurgent operations by the underground African National Congress that Mandela headed.

The congressmen arrived amid warnings that South Africa may be about to launch reprisal raids into neighboring black countries following a spate of land-mine and bomb explosions that have killed 14 whites in the past three weeks.

Local political commentators are also suggesting that Botha is unlikely to announce any significant modifications of apartheid when he opens the 1986 parliamentary session in Cape Town Jan. 31. Such changes were being widely predicted a few months ago but Botha has called a halt in the face of the continuing racial violence, the commentators say.

This circling of the wagons by the ruling white Afrikaner minority could present the congressmen with conflicting impressions of the effects of increased U.S. pressure on the Pretoria government.

Government officials and businessmen, including many white liberals, are likely to tell the congressmen that the pressure has caused attitudes to harden and that the best way to encourage change in South Africa -- as the Reagan administration has long advocated with its policy called "constructive engagement" -- is to try to coax rather than force South Africa toward change.

Most black community leaders, on the other hand, are expected to argue that the government's response to pressure is bravado, and that only increased pressure could induce white South Africa to abandon its system of minority rule.

The congressmen will be hearing plenty of both viewpoints during their four-day visit. Today, they saw the black township of Soweto and had a three-hour meeting with Foreign Minister R. F. (Pik) Botha. In addition to the session with President Botha Wednesday, they are to have talks with Black Affairs Minister Gerrit N. Viljoen and Constitutional Affairs Minister Chris Heunis, both key architects of the program for limited changes.

Later they are to see an array of black activists and their supporters.

South Africans tend to look inward in times of trouble, and few except the top leadership have shown much awareness of any potential this visit offers for influencing U.S. congressional policies toward this country.

Some newspapers have noted that Gray sponsored anti-apartheid legislation last year, and brief mention has been made of Fauntroy's record of opposing apartheid. But for most of the newspapers here, this is just another visit by a group of hostile foreign politicians.

"Yankee Pilgrim Trips to South Africa Begin," ran the inside-page headline in the main Afrikaans-language daily, Beeld, today, over a report of how a succession of other politicians and clerics was soon to follow.

[In Washington, the State Department announced that Assistant Secretary of State Chester A. Crocker will visit Angola Wednesday in a renewed effort to achieve independence for Namibia, which is now under South African control. He is to visit South Africa later in the week.]

Business Day, the country's leading financial daily, did not report the arrival, but gave front-page display to news of the imminent arrival of Fritz Leutwiler, the Swiss banker who is trying to reschedule South Africa's foreign debt.

For many South Africans, the financial crisis caused by American banks refusing to roll over $14 billion in short-term loans is more important than the limited sanctions -- mainly reinforcing earlier strictures on U.S. exports -- imposed by President Reagan in September when congressional action was imminent.

The financial crisis resulted in the freezing of all debt repayments and the imposition of foreign exchange controls.

The other visiting congressmen are Edolphus Towns (D-N.Y.), Charles A. Hayes (D-Ill.) and Lynn M. Martin (R-Ill.).