Argentina is asking foreign banks for a waiver on an international loan agreement so that it can borrow money early to pay overdue interest on other loans, New York banker William R. Rhodes said yesterday.

Rhodes, senior vice president of Citibank, said that the group of 11 banks advising Argentina had agreed that Argentina should ask for the waiver on the existing bridge loan.

If Argentina brings the interest payments up to date, it will clear the way for final agreement on a new, medium-term credit of $1.5 billion. This credit would extend for five years with a grace period on repayment of two years. Rhodes said he expected it to be approved by the end of July.

Argentina also wants to stretch out over seven years all its foreign debts due to be repaid in 1982 and 1983. It will negotiate this refinancing separately with all the different agencies involved, Rhodes said.

A banker who did not want to be named said that the main snag in these negotiations is that Argentina's bankruptcy code appears to favor domestic creditors over foreign lenders if an Argentinian concern goes bankrupt. The banker said that the Argentinian government had advised foreign banks that legislation was under way to change the code.

Argentina is one of the three biggest borrowers in Latin America, with close to $40 billion of foreign debts. It has a loan agreement with the International Monetary Fund that provides for restrictive economic policies in exchange for IMF money. It is presently in compliance with the IMF conditions.

Meanwhile Brazil, with more than $80 billion of overseas debts, is still bogged down in negotations with the IMF on a redrafting of its loan agreement. Brazil has failed to meet the IMF conditions and has therefore been unable to draw the latest installments of separate three year loans from the IMF and from commercial banks. So far the Brazilian government has not come up with significant new austerity measures that might satisfy the IMF and its other creditors.

A new factor complicating the negotiations is that President Joao Figueiredo is ill and will be coming to the United States for heart surgery in the next few weeks. Informed sources in Washington and Brazil said that this would likely hold up agreement between Brazil and the IMF.

Planning minister Antonio Delphim Netto, a key player in the economics team, does not get on well with the vice president, who will stand in for Figueiredo, sources said.