A Reagan administration plan to restructure up to $16 billion of military loans to economically ailing foreign countries "should not be implemented without specific statutory authority" from Congress, according to a General Accounting Office report.

The GAO report, while acknowledging that the law is ambiguous on the matter, reflects the concerns of some members of Congress. They think that the administration plan would force the government, at worst, to lose hundreds of millions of dollars in forgiven debt or, at best, be at "substantial risk" of losing some fraction of that sum in reduced interest payments to the Federal Financing Bank.

Treasury lawyers contend that the administration plan does not require new legislation.

The plan to ease the burden of high interest rates on foreign borrowers was hatched over the past couple of years, according to an administration official, as countries such as Egypt, which holds $4.5 billion of outstanding loans borrowed through the late 1970s at 9 to 16 percent interest, asked the United States to refinance their loans at current interest rates.

"We were trying to find a way to give countries short-term relief that would allow us to collect our loans in full," said an administration official who asked not to be named.

The administration came up with two ways of refinancing such countries: one would not recover the loans in full, which officials indicated was somewhat undesirable for the United States; the other would cost foreign countries much more in the long run, making this option undesirable for them.

The first method provided for the debtor country to pay off the principal of the loan plus accrued interest without paying a penalty, in exchange for the Defense Department's dropping its guarantee of the loan. With this option, the government would lose the value of future interest on the loan.

The second method involved "partial capitalization" of interest on the loan: In exchange for paying a lower interest rate now, the foreign country could delay paying off that interest to a final, inflated "balloon" figure when the loan came due.

"We have concluded," the GAO said, "that under the options available to debtor nations under the administration's debt restructuring proposal, the United States will incur either a financial loss or a substantial risk of significant financial loss."

Rep. David R. Obey (D-Wis.), who requested the GAO report, agreed with the report's conclusion that capitalizing the loan and pushing off enlarged interest payments to some future date means the administration eventually may need to forgive the loan altogether.

"If the administration wants to {forgive the debt}, they should fess up and say so now, not eight years from now," said Obey, chairman of the House Appropriations subcommittee on foreign operations.

An administration official disputed that assessment. "If anybody along the way in the administration would have said that that was the purpose -- to defer {the debt} off to future administrations and Congresses so they would have to forgive the debt -- they never would have gone anywhere," he said.

Egypt decided it was no longer interested in the U.S. refinance scheme after it rescheduled its international obligations last spring with the Paris Club, the name given to the informal meetings of Western creditor governments that renegotiate "official" debts owed by developing countries to those governments or guaranteed by industrial governments. It is called the Paris Club because the ad hoc meetings are held at the French finance ministry.

The legal basis for the GAO report's claim that the administration should not unilaterally restructure foreign loans was that without specific statutory authority, no agent of the government may waive the country's contractual right to receive repayment of loans on a certain schedule and at a certain interest rate.

"We are aware," the report said, "of no such specific authority applicable to the loans here in question, and Treasury has brought none to our attention."

Obey said members of Congress recognize that cumbersome military debts need to be refinanced for friendly countries but think that the administration should do it "up front" and "responsibly."

The administration official said that members of the appropriations, authorization and budget committees were briefed about the proposal last December and January and that the reaction was mixed. Obey later asked the GAO to look into the matter.