The House Banking Committee yesterday rejected an attempt to block proposed legislation that would prohibit the Export-Import Bank from supporting any private business transactions with South Africa.

But the committee deferred until next Wednesday a vote on another proposal to prevent a ban on the bank's doing business in South Africa.

These actions came as the committee began considering legislation that would raise the Export-Import Bank's lending authority from $25 billion to $40 billion. The bank is a government-controlled and-funded agency whose purpose is to boost U.S. foreign trade by providing financing for overseas sales by American firms.

In approving the bill last week, a Banking subcommittee adopted an amendment offered by Rep. Paul Tsongas (D-Mass.).

It would bar any Export-Import Bank transactions with South Africa unless the president certifies to Congress that the white supremacist government there has made "significant progress" toward voting equality for its black majority.

Since 1964, the bank has been prohibited from making direct loans to South African purchasers of U.S. exports. However, the bank currently has about $200 million invested in insurance and other guarantees for loans obtained from private U.S. banks by South African purchasers of American goods.

If the Tsongas amendment remains in the legislation as finally approved by Congress, it would be a serious blow to South Africa's ability to buy American products. Several House sources said yesterday the South Africans have started a strong lobbying action aimed at deleting the amendment from the bill.

The first efforts surfaced yesterday when two members of the Banking Committee, Reps. Richard Kelly (R-Fla.) and Garry Grown (R-Mich.), offered counterproposals to the Tsongas amendment.

Kelly's amendment would have made the ban on loans to South Africa inapplicable as long as any communist nations receive aid from the Export-Import Bank. It was rejected 38 to 6, after an acrimonious exchange between Kelly and other committee members about the death of South African black civil rights leader Steve Biko.

Biko died last Sept. 12 while in police custody. Although a South African judicial inquiry found no wrong-doing, several independent observers have charged that the evidence points to his having died as the result of beatings by police.

When Kelly said South Africa had made advances in racial relations, Rep. Parren J. Mitchell (D-Md.) asked if they "include the murder of Steve Biko."

"They might if I knew all the circumstances," Kelly replied.

His answer caused Tsongas to ask the committee chairman, Rep. Henry S. Reuss (D-Wis.), to give Kelly the chance to retract his statement about Biko.

Kelly refused and said: "My colleagues have better succeess in determining the truth from the newspapers than I do. I will not condemn an ally and a nation that I admire for many strengths on the basis of what I read in a newspaper."

"If the reports are true," he added, "then I denounce any brutality or violation of human rights." But, Kelly contended, "we just don't have enough information to make a judgement" about the Biko case.

The amendment offered by Brown would strip the ban on bank aid to South Africa from the legislation. Brown said the Tsongas amendment was "totally ineffectual" because it would simply cause South African buyers to go elsewhere and thus punish only American exporters.

In postponing a vote on the Brown amendemnt, Reuss said, "I will oppose it because I don't think the U.S. government should make it easy for South Africa at a time when it clearly is not making progress toward racial equality."