Despite the Reagan administration's attempts to isolate the Sandanista regime, Nicaragua has managed to keep paying its international debt on time and has negotiated its first loan with U. S. banks since the overthrow of the Somoza dictatorship in 1979, Nicaragua's top foreign financial official said yesterday.

Edmundo Jarquin, head of Nicaragua's Fund for International Reconstruction, said the $30 million short-term loan, negotiated with a group of banks headed by San Francisco's Bank of America, is more important for its political than its financial implications, "because, for the first time since the revolution , we have gone back to normal banking relationships."

The loan is to be repaid in March from Nicaraguan export earnings.

Jarquin said that the loan -- which is supposed to be signed Monday -- should make other lenders more willing to deal with Nicaragua, even if President Reagan continues to apply pressure on banks and other nations and to veto Nicaragua's requests for aid from the World Bank.

Jarquin said that Nicaragua was close to closing a loan with several U.S. banks last spring until pressure from the Reagan administration killed the deal.

He said he believes the Reagan administration acquiesced in the $30 million credit because it is worried about the inability of many Latin American countries to repay their foreign borrowings and the chaos that would ensue if any country, including Nicaragua, defaulted. Jarquin said on Thursday that Nicaragua paid its banks the $40.4 million it owed them and would have made the payment even without the Bank of America loan. But he said the loan made it easier on the country.

Jarquin was here to sign a $34 million loan agreement with the Inter-American Development Bank. The United States does not have a veto in the Inter-American bank, as it does in the World Bank, where Nicaragua has been unable to get development loans.

Jarquin said that, despite total withdrawal of financial assistance by the Reagan administration and its attempts to isolate the revolutionary regime, Nicaragua is one of the few countries in Latin America that continues to pay its debts on time. Others, such as Mexico and Brazil, must keep borrowing to pay the interest on their debts, he said.

But the terms on Nicaragua's debt are much easier than the terms on Brazil's and Mexico's debts. In 1980, for example, private banks reduced the interest on the $580 million Nicaragua owed them to 7 percent and permitted the country to defer paying any of the principal until 1986. Nearly all of the $1.6 billion debt inherited from Somoza was renegotiated months ago.

The $1.2 billion Nicaragua has borrowed in the three and one-half years since the revolution has been on almost concessionary terms.

Jarquin said that, although Nicaragua would prefer a friendlier relationship with the United States, the pressure the Reagan administration has exerted on the country has created sympathy elsewhere in the world. Hard-pressed Latin American countries -- including Argentina, Brazil, Peru and Colombia -- recently granted Nicaragua credits, and several major governments -- including Spain, West Germany and Japan -- renegotiated outstanding loans to the country on favorable terms.

When Reagan took office, he canceled all aid granted by the Carter administration to the Sandanista goverment. As a result, Jarquin said Nicaragua has refused to pay back or to try to renegotiate the tiny amount of official debt it owes the United States.