The Reagan administration, seeking to clarify an apparent misunderstanding, said yesterday that it did not agree last week to new concessions on $500 million owed by Israel to the United States but reminded the Israelis that they already have the right to postpone payment at the price of additional interest.

"I am not sure it is a question of the United States making an offer," State Department spokesman Alan Romberg said in reference to whether the administration had agreed to defer the quarterly payment due soon on the $9.6 billion Israeli debt owed for U.S. foreign aid. "There can be late payments, if that is viewed as necessary. That is a fact."

At the same time, Israeli diplomatic sources here, backing away from claims by officials in Israel that there had been a specific agreement, said Secretary of State George P. Shultz mentioned to Israeli Prime Minister Shimon Peres last week that postponing the debt payment would be one way of easing Israel's balance-of-payments problems.

However, the Israeli sources disputed reports from Israel that the Peres government has decided on that step. Instead, the sources said, deferral of the debt payment is one of several options under study by Israeli government financial experts, and the sources insisted that Peres' cabinet has not decided whether it will take that course.

From Jerusalem, Washington Post correspondent Edward Walsh reported that unnamed officials were quoted in the Israeli press yesterday as saying that, if Peres does opt for deferral, he plans to ask for about $700 million in additional U.S. aid early next year and then use $500 million of that to cover the delayed loan repayment.

Walsh also quoted Israeli officials as saying that, during Peres' U.S. visit last week, a secret agreement was made on deferring the payment. According to Walsh, the officials said Peres was forced to reveal the agreement to his cabinet after U.S. sources allegedly leaked details to a Radio Israel reporter.

These developments came after announcements Monday by Israeli cabinet members that, during the Peres visit here, the United States had offered to defer Israel's $500 million quarterly payment in order to help conserve Israeli foreign currency reserves. U.S. officials immediately replied that, while that had been one of several contingencies discussed with Peres, no decisions had been made.

During a flight to Toronto Monday, Shultz told reporters, "We have discussed various ways in which any potential problems might be met. And, of course, loan repayment is one way to get at cash flow. . . . "

Shultz also added, "My own opinion is that it won't be necessary to take such additional steps."

He noted that the United States is immediately making available $1.2 billion in economic aid of the $2.6 billion allocated for Israeli military and economic aid during the present fiscal year. He and other administration officials have said they believe that this amount, coupled with Israeli austerity measures, will be sufficient to ease Israel's balance-of-payments problems until early next year.

Yesterday, as questions persisted, Romberg and other officials expanded on earlier administration explanations by saying Israel did not need explicit U.S. approval for late payments.

They said that several countries, most recently Egypt and Morocco, have temporarily fallen behind on payments over the years and that a system has evolved for dealing with such situations. They described it this way:

Loans for foreign military-sales credits are made by the Federal Financing Bank, part of the Treasury Department. When a debtor fails to make a scheduled payment on loans, the bank collects the amount from the Defense Department, which has a special fund to cover such contingencies.

Collection of the delinquent amount then becomes the Pentagon's responsibility. The debtor government is obliged to pay not only the amounts originally due on principal and interest but also additional interest, determined by a special formula involving the amount and the length of time it is past due.

Israel traditionally has paid foreign debts promptly. As a result, Israeli sources said, the Peres government is concerned that postponing the upcoming payment to the United States would be counterproductive to Israel's hopes for long-term financial recovery because it would damage the nation's international credit standing.

U.S. officials, while reiterating Shultz's argument that Israel can avoid a serious short-term reduction of reserves without postponing the U.S. loan payment, said the decision is up to the Israelis. These officials also stressed that, if Israel defers payment, that should not be interpreted as a long-term move toward U.S. rescheduling or forgiveness of the $9.6 billion owed by Israel.

Walsh quoted a senior Peres aide as saying that postponing the $500 million repayment was first raised Oct. 7, Peres' first day in the United States, by two unidentified members of Congress, who met with him in New York.

The official said that Peres initially was reluctant to consider the idea because of the potential effect on Israel's credit rating but that the members of Congress argued that it was the only way the United States could provide immediate help before Congress reconvenes in January.

Then, the official added, the idea of postponing repayment was raised by Shultz at a breakfast with Peres just before Peres met with President Reagan at the White House Oct. 9. The official said he did not know if the subject came up in the meeting with Reagan but added that Shultz described it as a decision already made by the administration.

After Peres accepted, the official continued, the two sides agreed to keep the postponment secret. But when Radio Israel broadcast details as Peres returned home last Saturday, Peres felt that he had no choice other than to inform the cabinet, the official said.