U.S. bank loans to South Africa have declined recently in both volume and length, after showing a fairly steady increase during the past four years, according to U.S. Federal Reserve figures.

Political unrest and continuing recession in South Africa also have led a number of banks to announce more restrictive lending policies to the South African government, banks and businesses. Citicorp, Morgan Guaranty Trust, Chase Manhattan and the Bank of Boston are among the many that have announced new policies.

"I don't see a wholesale defection from South Africa, but the decline in loans has got to reflect the banking community's concern about continued deterioration of the economic and political situation in South Africa," a banking official said.

The tighter credit to South Africa may cause its economy some problems. South Africa is experiencing its worst recession in years, and its foreign borrowing requirements have risen sharply.

The Bank for International Settlements reported that South Africa's total net borrowing grew 25 percent from 1983 to 1984, when it stood at $15 billion.

Of that total, U.S. banks accounted for $4.9 billion in September 1984.

But in the months following September, a decline started, and by March of this year, the total was $4.1 billion, according to the Federal Reserve's Country Exposure Lending Survey. This represents a drop of 16.3 percent.

During the same period, the number of loans issued for periods longer than five years fell from $181.3 million to $57.8 million.

The decline in loans is significant, because the South African government already has suffered a drop of 11.5 percent in direct investment by the United States between 1981 and 1983.

"If South Africa experiences a drop in investment and a drop of bank loans, that is going to have some sort of detrimental effect, but nobody can say to what degree," said one banking official, who asked not to be named. "You can't just assume that other countries will leap in if the U.S. retreats."

Loans are dropping for several reasons, according to David Hauck, head of the South Africa Review Service of the Investor Responsibility Research Center in Washington.

"Part of the reason for the drop is that the South African government is importing less, and the reason it is importing less is because the economy is doing badly, and the rand is so weak," he said.

But the drop can also be attributed to bankers' fears about the stability of the South African economy, as well as the political and social unrest in the country. Since September 1984, about 600 persons have died.

During the past few months, several major U.S. banks that had not banned loans to the South African government after unrest in the 1970s reversed their policies and announced a ban. Among these are Citicorp, NCNB in North Carolina and Morgan Guaranty Trust.

Then some banks announced they would no longer lend to the private sector in South Africa. This move is particularly disturbing to South Africa because U.S. lending to the private sector had been escalating in recent years, offsetting the decline in loans to the public sector.

Wayne Taylor, manager of media relations for the Bank of Boston, said the bank had announced in March that it would make no more loans to banks and businesses in South Africa because it opposes the strict system of racial segregation known as apartheid. The bank's loans to nine South African banks had averaged $75 million in 1984 and would run out by 1986.

In addition, he said, "The social and political unrest in South Africa calls into question the economic and political stability of the South African government, and that is a business matter of concern to us."

On July 31, reports circulated that Chase Manhattan had told South Africa it would no longer make loans to the private sector and would not renew existing ones.

Steven Rautenberg, director of public relations at Chase Manhattan in New York, said Chase "would not confirm nor deny" the reports. Other banking sources said Chase officials had told them the reports were true.

In response to the reports, the rand plummeted almost 10 percent overnight, apparently because of fears that the South African government would impose currency controls to stem capital flight, according to Business Week magazine.

Chase's loans to the private sector are reported to be relatively small -- about $400 million -- but some financial experts think that Chase's prestige in banking circles puts pressure on other banks to do the same.

But some banking officials said they did not think the Chase action would have a major effect on their policies.

"We already made our decision before the Chase move," said Mary Ullrich, vice president of public relations at Harris BanCorp., which owns Chicago's third-largest bank.

"Early this year, we took South Africa off our approved country list, and we now have no exposure there at all," said Ullrich.

"We decided as part of our normal country evaluation process that the economic and political situation in South Africa did not warrant any exposure," she said.

Several other banks, including Norwest Corp. in Minneapolis, said they were no longer lending to the South African private sector.

Harold Webster, a spokesman for Norwest, said the bank announced its decision Aug. 1.

"Based on the recent deterioration of economic, social and cultural conditions, we decided it was no longer wise to lend money to South Africa," Webster said. His bank holds $10 million in loans to private companies.

Some banks reported that they would not stop lending to the South African private sector, but a number said they are lending less and less money to this sector.

"Our exposure in South Africa has declined dramatically," said Russell Page, senior vice president of NCNB in North Carolina. In 1983, NCNB had $217 million outstanding in loans, made mostly for trade financing. In 1984, the amount was $130 million, and as of now, it is $101 million, he said.

"With the current climate and economic conditions in South Africa, I'd expect the decline to continue," Page said.

NCNB also has adopted a policy of prohibiting loans to the government of South Africa and its agencies, Page said. In 1984, outstanding loans to the government amounted to about $7 million.

But the country's two largest bank holding companies, Citicorp and BankAmerica, said they are not planning to change their policies and will continue to lend to the private sector.

Peter Magnani, a spokesman for BankAmerica, said the bank would continue to finance short-term trade and commercial and industrial projects.

Magnani confirmed that BankAmerica has one outstanding loan to a state-owned corporation in South Africa, but he refused to disclose the amount. He added that the bank adopted a policy in 1980 not to lend any more money to the South African government or state-owned companies.

Citicorp said it, too, would continue to provide loans to the South African private sector. Bill Koplowitz, spokesman for Citicorp, said the bank's "disciplined, constrained" involvement would be of greater benefit to South African blacks than withdrawal.

Like Citicorp, Morgan Guaranty will continue to make loans to the private sector, said a Morgan vice president, Frederick Allen.

But both Citicorp and Morgan Guaranty did make some recent changes in their policies toward South Africa. Until this spring, Citicorp had one outstanding loan of about $20 million to the South African government. In March, Citicorp announced that the loan would be liquidated, and no new loans would be made to the South African goverment. Citicorp's previous policy was to approve loans to the government that were of "direct and unique" benefit to the black population.

Citicorp's new policy came amid moves by New York officials to cut off millions of dollars in city business from banks and corporations with ties to Pretoria.

In early March, Morgan announced a new policy toward South Africa, saying it would not make any new loans to the government or state-owned agencies "until real progress has been made toward the improvement of political, economic and social conditions for blacks and other nonwhites in South Africa."

Morgan has three government loans on the books, said Allen, who described the amount as "modest" but would not give a precise figure.

The pattern of declining loans during periods of social and political unrest has been observed before in South Africa.

A report done by the Investor Responsibility Research Center in December 1984 said a decline in lending followed widespread civil unrest in South Africa in the late 1970s. For instance, total loans fell from $2.2 billion in 1978 to $1.3 billion in 1980, and then began increasing steadily during the next several years, when conditions were calmer in South Africa.