The United States believes that Argentina's intransigence is scuttling the last hope of resolving the Falklands crisis without further bloodshed and that Britain probably will recapture the islands after "new and terrible fighting," a senior Reagan administration official said yesterday.

The official, who declined to be identified, warned that Britain already has offered almost all the concessions it is able to make and that any hope of averting renewed warfare depends on Argentina dropping its thus far inflexible insistence that its sovereignty over the Falklands must be acknowledged as a condition of any settlement.

"Our experts feel it is important not to underestimate the British resolve in this matter of principle," he said. "We believe the British will do whatever it takes to recapture the islands if the Argentines do not withdraw. The lull in the South Atlantic could give way to new and terrible fighting . . . and it's probable, given their assets, that the British would prevail in the end."

The official gave this assessment as President Reagan publicly expressed concern about Britain's announcement that it would regard as hostile any Argentine warship or military aircraft operating more than 12 nautical miles from the Argentine mainland.

In a brief talk with reporters, Reagan said, "I am concerned, of course. I don't want violence to break out again."

However, the senior official, while insisting that the United States does not know if or when Britain plans new military action, hinted strongly that the last hope for a negotiated settlement rests with the mediation effort about to be tried by U.N. Secretary General Javier Perez de Cuellar.

The United States, the official said, hopes the effort succeeds and will do everything it can to help. But he added: "It's hard to be optimistic until Argentina's position is made clear . . . . If Argentina continues to insist on sovereignty or on decoupling a cease-fire from withdrawal of its occupation forces , there will be no progress.

"The problem continues to be Argentina. In the contemporary sense, we feel that the countries that want peace must make clear to Argentina that there must be a withdrawal and cease-fire without any precondition of sovereignty determined in advance."

He said it was Argentina's refusal to accept that principle that caused the failure first of the peace plan put forward by Secretary of State Alexander M. Haig Jr. and then this week of a joint initiative by the United States and Peruvian President Fernando Belaunde Terry.

He confirmed British Foreign Secretary Francis Pym's public account yesterday of the details of the U.S.-Peruvian plan and said it was true, as Pym asserted, that Britain had agreed to accept it, only to encounter a rejection by Argentina.

The official said both the U.S.-Peruvian initiative and Haig's earlier plan had been "fair and reasonable," and actually favored Argentina's goals. Yet, he added, Argentina had turned them down, "while the British were willing to compromise and had gone well beyond what they wanted to in terms of making concessions that weren't easy for them to swallow."

He blamed Argentina's stance on the inflexibility of the military leaders who control that country and their belief that time is on their side.

In particular, the official said, the Argentines seem to believe that growing pressures from other countries, concerned about the dangers of widened warfare, will force Britain to back away from confrontation and permit Argentina to remain in control of the Falklands.

For that reason, he added, the United States fears that Argentina's acquiescence in U.N. mediation is a stalling tactic aimed at letting these pressures continue to build.

The United States, he continued, views that as a misreading of the situation because the problems of maintaining the British armada in the South Atlantic, as supplies dwindle and winter weather sets in, are rapidly eroding Britain's ability to hold back from new military action.

"At this juncture, the time is most propitious for a settlement," the official said. While making clear that the United States isn't very optimistic about the chances, he conceded: "Maybe a U.N. venue will permit Argentina's friends to apply greater pressure than Peru was able to do alone. We hope that will be the case."

He acknowledged that the United States, following the failure of its two initiatives, is not now playing an active mediating role.

And he added that, while Washington stands ready to help, it will continue to support Britain because of historic Anglo-American ties and because of the need to adhere to the principle that "aggression cannot be seen to be rewarded."

The official admitted that this position has been costly to U.S. influence in Latin America, whose countries have felt compelled to show solidarity with Argentina.

But, he argued, if the United States allows the principle of peaceful settlement of disputes to be violated, there could be a chain reaction of resorts to violence in other Western Hemisphere disputes such as the Beagle Channel issue between Argentina and Chile, Venezuela's territorial claims against Guyana, Guatemala's desire to annex Belize, and Nicaragua's alleged attempts to interfere politically in the affairs of its neighbors.

Asked whether Britain has a deadline before it returns to military action, the official replied, "I don't know what their plan is in that regard. They have been committed from the outset to the minimal use of force, provided their security zone around the Falklands is not violated. If it is, they'll take action.

"But we are not privy to their plan. We haven't asked them, and they're not sharing it with us."

The official also denied increasing rumors that the United States has been asked to provide the British armada with all manner of assistance ranging from food and fuel to sophisticated radar surveillance planes and missiles.

"They haven't asked for anything," he said.

However, there were wire service reports last night that Britain has asked for American tanker planes to refuel its aircraft. U.S. officials said no decisions had been made, and one suggested it might be a "tough call."

After the United States declared its support for Britain last week, Pym conferred here with senior U.S. officials about possible U.S. aid. The only U.S. help to Britain thus far is the fueling aid provided by the U.S. base on Ascension Island along the route toward the South Atlantic.