A senior Argentine Army official and chief aide to President Leopoldo Galtieri said today that any peace agreement between Britain and Argentina should contain "a sure and guaranteed route" for the recognition of Argentine sovereignty over the Falkland Islands "within a reasonable time."

The statement by Gen. Hector Norberto Iglesias was delivered in writing to The Washington Post after an interview. It indicated that the ruling military junta, despite shifts in its negotiating stance, has not altered substantially its position on control over the disputed South Atlantic territories in this week's talks with United Nations Secretary General Javier Perez de Cuellar.

Britain demands that a peace agreement, arranged by the United Nations, must include a clause that will not prejudge the outcome of talks on the future of the islands.

Iglesias said in the written statement, "We have said that we pursue only one objective: restore the Malvinas Falklands to our national patrimony, place those territories under our sovereignty. We are demanding, therefore, that whatever agreement is arrived at for a peaceful solution should constitute a sure and guaranteed route so that sovereignty will be total and full within a reasonable time."

Iglesias, who has the title of general secretary of the presidency, is a top aide to Galtieri and participated in the drafting of Argentine proposals for the U.N. mediation. He represented Galtieri in talks with Peruvian President Fernando Belaunde last week and participated in the earlier talks between the Argentine government and U.S. Secretary of State Alexander M. Haig Jr.

Responding in writing to a series of questions, Iglesias submitted a five-page statement to The Post saying, in part, "We do not demand a previous and formal recognition of our rights. We have made very costly efforts to arrive at a formula that gives certainty to their exercise, that effectively leads to our dominion and that does not declare it prima facie."

This attempt to guarantee that negotiations will lead to Argentine sovereignty over the Falklands, without a previous formal recognition by Britain, was the basis of most of the talks between Argentine officials and Haig last month.

Any movement in Argentina's position since then has concerned what government sources describe as a willingness to be more flexible about what constitutes a "guarantee" for Argentina in any settlement. In essence, this means that the Argentines are willing to be flexible about terms of negotiations on a range of issues, perhaps including the timing and conditions of their ultimate sovereignty. But in any negotiations Argentina foresees, regardless of the public face or interpretation Britain or others may put on them or of the actual terminology of a guarantee, sovereignty itself will never be in doubt.

Iglesias also revealed that while Argentine officials considered the possibility of a militant British response to their invasion of the Falkland Islands April 2, the "intermediate hypothesis" had been that the United States would restrain Britain's reaction, forestalling a military conflict in the South Atlantic.

Iglesias also blamed the failure of peace proposals by Peruvian President Belaunde last week on Britain's sinking of the Argentine cruiser General Belgrano, which occurred just as Argentina was considering the proposals.

He also denied that Argentina is seeking new weapons supplies from other nations, and he asserted that Argentina is intending to continue playing a strategic role in the West when the South Atlantic crisis ends.

Iglesias' interview and statement come at a time when Argentine officials have said they would like to persuade international opinion, particularly in the United States, that Argentina has been flexible in seeking a peaceful solution with Britain.

Government sources here have said that Argentina believes that international opinion is beginning to turn in its favor in Europe and in the United States. They have sought this week to counter statements Friday in Washington from a senior State Department official who accused Argentina of intransigence and holding up an agreement with its insistence on sovereignty rights.

Iglesias said Argentina is "deeply and totally convinced that this conflict can only have a true and stable solution through a diplomatic accord grounded in reasonable and realistic bases."

But, he said, "What is certain is that Great Britain always avoided or refused to discuss the underlying question."

For Argentina, that question is sovereignty, which Iglesias defined as "the practical exercise of our rights over the islands as an integral part of national territory."

"The same added suit has been primary on our part during all of the negotiations that have been held," Iglesias said. "We have had an open position."

Since nearly the beginning of the negotiations, Argentina has maintained that a peaceful solution could be arrived at in two different ways.

The first route was indicated by Argentina last week when it said that the beginning of negotiations should be an immediate cease-fire without other conditions. Britain responded to this statement by saying that a cease-fire must be accompanied by the withdrawal of Argentine troops.

In answer to this statement, Argentina said that it would only conduct such an unconditional troop withdrawal if Britain simultaneously and formally recognized Argentine sovereignty.

In the other route, Argentine officials agreed early during negotiations with Haig that the sovereignty question could be set aside if both future negotiations and a temporary administration for the island contained implicit assurances of eventual Argentine control.

This dual position was made clear with the release here of a letter from Foreign Minister Nicanor Costa Mendez to Haig on April 29 that was Argentina's answer to the plan proposed by Haig before the breakdown of U.S. diplomatic efforts.

In that letter, Costa Mendez noted that the "fundamental base" for the talks with Haig had been "recognition of sovereignty and the regimen of a provisional administration," and added "certainly both topics are intimately related."

"In the event that the dispositions related to the recognition in our favor of sovereignty turn out imprecise," the letter said, "it makes it necessary for us for the establishment of mechanisms that give us greater facilities for the administration of the islands."

"Inversely, if it remains clear that Argentine sovereignty will be recognized in the last instance, it increases our flexibility in the substance of the provisional administration," the letter said.

During talks with Haig, the assurances sought by Argentine officials in the event sovereignty was not recognized immediately included a fixed date for the end of negotiations and an automatic reversal of sovereignty to Argentina in the event an agreement was not reached.

Argentine officials also said they could be satisfied if during talks, Argentina held a clearly superior position over Britain in the islands, and they proposed such possible arrangements as having an Argentine provisional governor and a British vice governor or equal administration with an Argentine peace force.

Iglesias' statement indicates a slight shift in Argentina's talks with Perez de Cuellar by affirming that Argentina could accept temporarily an equal position with Britain in the administration of the islands "to the extent that the peaceful solution fulfills the precautions" he had previously referred to.

Other government sources have said this week that Argentina has sought to show more flexibility and make its position easier for Britain to accept by proposing a series of new "precautions" for the negotiations, while agreeing for the moment on an equal role for Britain in a U.N. administration for the islands.

For example, these sources say, Argentina is still asking for a fixed end to the negotiations, but it is no longer demanding that sovereignty automatically revert to Argentina in the event no agreement is reached by the deadline.

Instead, government sources said, Argentina is now seeking only a sort of tie-breaking mechanism that will assure the decision is reached in a set period. Argentina is also willing to extend the time limit for negotiations beyond the Dec. 31, 1982, date cited to Haig, sources have said.

Iglesias' statement severely criticized Britain for "obstructing all the possibilities for a peaceful solution up until this time." It cites attacks by Britain that have coincided with the negotiations.