American banks and investors have helped South Africa's white minority government strengthen apartheid and are in effect "undermining the fundamental goals and objectives of U.S. foreign policy" in southern Africa, Sen. Dick Clark charged yesterday.

Speaking at a press conference in the Capitol, the Iowa Democrat, who is chairman of the Foreign Relations Subcommittee on African Affairs, called on the Carter administration to reinforce its strong public condemnations of South Africa's racial policies with concrete steps that "actively discourage American investment" in that country.

Clark asked the White House to withdraw U.S. government - sponsored financing for American exports to South Africa, and said that tax benefits should be canceled for American companies that contribute in any way to South Africa's separate development policies and to its continuing military buildup.

Other steps he advocated included the drawing up of official guidelines on investment and fair employment practices for the approximately 300 U.S. companies that have put at least $1.6 billion into South Africa over the past two decades.

Clark called his recommendations "moderate," saying that they fall between the growing calls for a complete prohibition on American business dealings with South Africa and arguments that the American business presence has helped chip away at apartheid's worst features.

While generally praising the Carter administration's strong verbal condemnations of apartheid, Clark added that the United States "could go a bit further" in using trade policy to "encourage" South Africa to alter its harsh system of racial segregation and political repression.

But his call for new action comes at a time of growing concern by the administration about continuing large U.S. balance of payments deficits and trade issues in general. Senior administration officials suggest privately that the White House is reluctant to seek trade and financial restrictions against South Africa in present conditions.

Clark's recommendations were contained in a report to the Senate Foreign Relations Committee that he released at the press conference. Giving one of the most detailed and critical analyses yet made of the American business presence in South Africa, the report charges that American money transfers have been "pivotal in directly assisting the South African government during its worst economic difficulties in the past and, if permitted, could do so in the future."

Loans from abroad "provided the margin of funds needed by South African in the 1974-76 period to finance its military buildup, its stockpiling of oil and its mayor infrastructure projects in strategic economic sectors," the report states. South Africa's total foreign debt at the end of 1976 was $7.6 billion, almost one-third of which came from American banks.

The $2.2 billion loaned by Chase Manhattan, Citibank, Manufacturers Hanover Trust, Bank of American and seven other banks "is roughly equivalent to the amount of foreign exchange required to cover South Africa's defense and oil imports costs for the same year," Clark's report added. International credit has "filled the gap" in modernizing South Africa's military forces as the deep recession that began in the country in 1974 has lingered.

Expanding on the report's assertion that "U.S. corporations operating in South Africa have made no significant impact on relaxing apartheid," Clark compared his plan of action to U.S. attempts to break the Arab boycott directed at firms that have significant dealings with Israel. "Multinational firms cannot be permitted to be tools of foreign governments committed to margins for U.S. companies that man [TEXT OMMITTED FROM SOURCE]

The main effect of Clark's proposals, of adopted, would be to cut profit margins for U.S. companies that manufacture and sell in South Africa. Those profit margins have already begun to drop because of the depressed economy.

The South African embassy issued a sharp reply to Clark's report, saying that "we do not share his belief that mass unemployment would help the black man, and respected black leaders in South Africa also reject his point of view with contempt."