Major U.S. banks increased their overseas lending by 20 percent in the year ended in June, according to a report released yesterday by the Federal Reserve Board, the Federal Deposit Insurance Corporation and the Comptroller of the Currency.

A survey of 143 U.S. banking organizations with sizable foreign operations showed that international claims of these banks were $266 billion in June, up from $221 billion a year earlier. Much of the increase was in short-term credits. Claims of more than one year's maturity rose by only $2.5 billion.

Loans to developed countries accounted for more of the overall increase than loans to Third World countries. But much of the lending to developed countries is interbank lending for very short periods. About three-quarters of the total reported international claims had a maturity of a year or less. But for most groups of countries, short-term claims were only about half of their total debts to the U.S. banks.

Worries about banks' overexposure in international lending center on their lending to those developing countries which already have a high level of indebtedness and which use a significant proportion of their exports to finance the interest payments on the loans.

Loans to developing countries which are not oil exporters rose by about 22 percent, according to the survey, to reach $66 billion at the end of June. The banks' claims on these countries account for about a quarter of their total international claims.

Poland, which recently asked the U.S. government for sizable loans, has $1.73 billion of outstanding foreign loans from large U.S. banks. This is the second-highest total in Eastern Europe, after Yugoslavia. More than half of the Polish loans are for between one and five years.

In Latin America, U.S. banks' claims on Brazil totaled $13.99 billion, while those to Mexico were $12.76 billion. Argentina was the next-largest borrower in that area, owing $5.6 billion to U.S. banks at the end of June.