Federal banking regulators reacted to the huge unpaid foreign debt saddling U.S. banks yesterday by imposing capital adequacy requirements on multinational banks. The standard will oblige them to raise more than $800 million in new capital in the near term.

The Federal Reserve Board and the Office of the Comptroller of the Currency declared that, effective immediately, the 17 largest bank holding companies will have to maintain primary capital equal to at least 5 percent of their total assets. Capital, as distinct from liabilities belonging to depositors, is put up by shareholders and acts as a reserve to absorb loan losses.

Five of the 17 banks currently do not have adequate capital. As of March 31, Citicorp had the lowest ratio of capital to assets, 4.67 percent. The others are BankAmerica Corp., 4.79 percent; Chase Manhattan Corp. 4.98 percent; Bankers Trust Corp., 4.79 percent; and Irving Bank Corp., 4.83 percent.

In the case of Citicorp, bringing the ratio up to the minimum would necessitate raising $250 million in new capital. A Citicorp spokesman said yesterday that the company opposes fixed limits, but did not believe raising the money posed any significant difficulties. Jack Ryan, the Fed's director of the division of banking supervision and regulation, said he thought most of the banks would be able to raise their ratios within 12 to 18 months.

Two years ago, the Fed set even stiffer capital adequacy requirements for regional and community banks. The big banks succeeded in persuading the Fed to exempt them because they always could raise money in the capital markets if needed. "I didn't think that day would ever come," said Fed Governor Charles Partee to Chairman Paul A. Volcker. "You did."

The day of reckoning came in the aftermath of problem loans made to Third World countries, particularly Mexico, Brazil and Argentina. The nine largest U.S. banks lent those three countries alone a total of $30.5 billion, or more than their total capital of $27 billion. One-third of their capital was put in jeopardy when the countries, unable to repay, asked for a one-year moratorium.

The 17 multinational banks have improved their capital positions in recent months as the stock market has boomed. The average ratio, which stood at 4.63 percent 15 months ago, has climbed to 5.35 percent. Nevertheless, both houses of Congress are in the process of approving curbs on banks' ability to make foreign loans in exchange for approving increases in the U.S. contribution to the International Monetary Fund.

Last week, the Senate passed a bill mandating special reserves to cover for problems loans and also requiring banks to increase their capital. An even tougher bill is awaiting House action. In a letter to House Banking Committee Chairman Fernand St Germain (D-R.I.), Volcker said the Fed's rules "will make a major contribution to the objectives we seek through this legislation." St Germain termed the rules "a very welcome step forward," although he said he would not pass judgment on the specific level adopted.

Ryan said yesterday that, if the banks do not achieve the minimum ratio, "They might not get the acquisitions they seek." The Fed also acted to expand the definition of secondary capital to include unsecured long-term debt issued by the parent company and its nonbank affiliates. Ryan characterized this as a way to give institutions greater flexibility in meeting the goals and in gaining access to capital markets.