South Africa's violence spilled over into white residential areas for the first time with clashes near Cape Town and East London last night and today.

In attempting to deal with the economic problems accompanying this country's political violence, the Pretoria government today also challenged the U.S. Congress, saying proposed economic sanctions would slow progress to change the system of racial separation known as apartheid.

In the United States, banks have severed key short-term credit links covering trade with South Africa following Pretoria's declaration of a four-month debt moratorium, authoritative U.S. banking sources said today. This threatened to disrupt trade between the United States and South Africa and deal a blow to the white-minority government's efforts to restore confidence in its beleaguered economy.

In the spreading violence, mixed-race youths attacked white homes in a suburb near Cape Town last night and were met by residents who opened fire on them, wounding two. Two highways traversing white suburbs of Cape Town also were closed today after black youths stoned cars.

On the outskirts of East London, a port city 800 miles east of Cape Town, a mob of blacks threw gasoline bombs and stones today at two white homes in the suburb of Amalinda. The homes are about two miles from the black township of Duncan Village.

The violence in the Cape Town and East London areas caused a ripple of anxiety within the white community, and gunsmiths in Cape Town reported a rush to buy pistols and shotguns today.

Despite the nervousness, experienced observers said they doubted whether these incidents indicated the start of a new phase in the violence aimed at attacks on whites.

They noted that both in Cape Town and East London, the black and Colored townships are situated much closer to the white city than is usual in South Africa.

Particularly in Cape Town, the segregated zones are close to each other. The city is built on a narrow peninsula, with its black and Colored townships less than a mile from some white suburbs, compared with distances of 10 or more miles from other large cities. There are some Colored suburbs within that city, but not elsewhere.

This means that when racial violence breaks out in Cape Town, it is more likely to spill over into white areas than is the case in other cities. The highways leading out of the city, which pass close by three big black townships, are also vulnerable to stoning attacks.

In Washington, the State Department said South African police have used "excessive force" to maintain order and also criticized the recent call by the African National Congress for greater violence as "irresponsible."

Discussing allegations of police brutality, spokesman Bernard Kalb repeated U.S. calls for restraint on the part of South African authorities in protecting the rights of all citizens of that country.

["The use of excessive violence has contributed to the increased level of violence," he said. "Violence in South Africa is at such a stage that use of force is unfortunately more and more common. These acts must stop."]

In a move aimed at influencing next week's vote in Congress on measures to enforce sanctions against South Africa, the Pretoria government released a brochure designed to show how sanctions would hurt South Africa's black neighbors.

The 20-page brochure, to be distributed through South Africa's embassies abroad, stresses the economic interdependence of countries in the region and notes the number of blacks from neighboring states who work in South Africa, as well as the interlinking of transport systems and electric supply networks.

"Let us be frank; our neighboring states will suffer before we do," said Deputy Foreign Minister Louis Nel, who released the brochure at a news conference in Pretoria,

"Those [sanctions] measures will have an impact on the whole of southern Africa, and South Africa will be better able to absorb them than its neighbors," Nel added.

"The choice is between sanctions on the one hand and political, social and economic progress on the other," Nel said.

The first attack on a white suburb occurred last night in a residential area called Kraaifontein, which is about half a mile from a Colored, or mixed-race, district on Cape Town's northern outskirts.

Reporters who were there said rioting, which had been going on for three days in a number of Colored districts around South Africa's southernmost city, spilled over into Kraaifontein late last night when a group of about 100 mixed-race youths threw gasoline bombs and stones at some of the white houses.

The two Cabinet members primarily responsible for security, Law and Order Minister Louis le Grange and Defense Minister Magnus Malan, flew to Cape Town as street battles between rioters and police continued for the fourth consecutive day and two more Colored men were shot dead in the townships.

Residents said Ebrahim Carelse was visiting neighbors in Elsies River when a policeman who had followed him kicked open the door and shot him in the head with a shotgun.

Jonathan van Wyk, 19, a Colored student, was shot dead when police opened fire on a crowd that had stopped a bus and stoned it.

Le Grange told reporters that the situation in Cape Town was "not satisfactory," but he said he believed that there were signs that the unrest was "leveling off" in other parts of the country.

Earlier today, the body of a 15-year-old boy who disappeared Tuesday was found with gunshot wounds. Police confirmed that he had been shot during antiriot operations. This brought the death toll in this week's Cape Town unrest to 32.

Police broke up groups of students gathered at schools in the colored townships several times, and three television reporters were beaten with batons and plastic whips as they tried to film one of the incidents. A television camera was broken. Later, police ordered journalists out of the townships.

In another development today, a court ordered Gencor, one of South Africa's major mining houses, to reaccommodate 53 miners out of a total of 967 that it has dismissed at its Marievale gold mine for striking Monday and Tuesday in support of a demand for higher wages.

The National Union of Mineworkers sought an urgent court order stopping the company from sending the dismissed strikers back to tribal "homelands" and replacing them with freshly hired workers.

In an affidavit to the court in behalf of the 53 miners, the union's secretary, Cyril Ramaphosa, asked that their expulsion from the mine premises be stopped while the union challenged the legality of the dismissals in a special industrial court Friday.