The United States, charging that Argentina has been unreasonable and intransigent, yesterday abandoned its even-handed approach to the Falkland Islands crisis by imposing military and economic sanctions against Argentina and promising to "respond positively" to British requests for military aid.

Secretary of State Alexander M. Haig Jr., who announced the policy shift, stressed that there will be "no direct U.S. military involvement" if further fighting breaks out between Britain and Argentina.

But Haig, blaming the breakdown of his mediation efforts on "Argentina's failure to accept a compromise," said President Reagan had decided "we must take concrete steps to underscore that the United States cannot and will not condone the use of unlawful force to resolve disputes."

Although Haig sought to be relatively diplomatic in his references to Argentina, Reagan was much blunter in a talk at the White House with midwestern editors and broadcasters. The president, referring to Argentina's invasion of the Falklands on April 2, said:

"We must remember the aggression was on the part of Argentina in this dispute over the sovereignty of that ice-cold bunch of land down there, and they just finally resorted to armed aggression, and there was bloodshed. I think the principle that all of us must abide by is: armed aggression of that kind must not be allowed to succeed."

A senior administration official acknowledged later that the steps ordered by Reagan, particularly in regard to sanctions, will have little practical impact on Argentina's military or economic situation. He said they are intended as an "essentially political signal" that might jolt Argentina's military government into taking a more flexible attitude that would permit resumption of negotiations.

Specifically, the president ordered suspension of all military exports to Argentina, withholding certification of Argentina's eligibility to buy American military equipment and the suspension of new credits and guarantees by the Export-Import Bank and the Commodity Credit Corp.

None will have much effect, because Argentina has been able to make only marginal military purchases in this country since 1977 and does not count the United States among its most important trading partners.

However, the U.S. promise of "materiel support" to Britain might prove of great significance if, as American officials fear, the British armada's present proximity to the Falklands leads either side into actions that cause the situation to explode in heavy fighting.

The British fleet, operating 8,000 miles from home, has its supply lines stretched perilously thin, and the United States could provide fuel and other supplies far more quickly than they could be obtained from Britain.

Defense Department officials said yesterday that the form U.S. aid might take is "very ill-defined" and will depend on whether fighting breaks out.

The senior official, while declining to discuss specifics, said any U.S. assistance would be "in the context of historic, ongoing relationships with Britain" and specific agreements existing between the two governments.

That appeared to suggest that the United States would continue to afford British ships and planes refueling facilities at the American base on Ascension Island in the Atlantic between South America and Africa and also would share intelligence and reconnaissance information.

A hint that Argentina might be backing away from a military showdown came from Foreign Minister Nicanor Costa Mendez, who said at the United Nations that his government was willing to abide by a Security Council resolution calling for withdrawal of Argentine forces from the islands, an interim administration acceptable to both sides and negotiations for a long-term resolution of the islands' status.

The senior U.S. official said the Argentines would have to be more specific before there was any way of telling whether Costa Mendez's statement was a positive sign. However, the official noted that, during Haig's three weeks of shuttle diplomacy, the Argentines informally told him the same thing several times, but always attached the qualification that the end result must be acknowledgment of Argentine sovereignty over the Falklands--a precondition unacceptable to Britain.

The official also took exception to Costa Mendez' assertion at the United Nations that Argentina had not rejected the U.S. peace proposals. but had merely made "observations that do not constitute a rejection."

The official said Costa Mendez' statement involved "a subjective bias," and added that there was "a very, very clear turndown" in the written reply that the United States received from Buenos Aires Thursday.

The official also sketched a chronology of the U.S. mediating effort, which he said began as the result of "urgent requests from the very highest levels" of both governments and that caused Haig to spend almost three weeks in intercontinental shuttling between Washington and London and Buenos Aires.

In pursuing his go-between role, the official said, Haig found the fundamental sticking points involved Britain's insistence that the wishes of the Falklands' residents be given "high precedence" in any solution and Argentina's refusal to consider any outcome that did not lead to acknowledgment of its sovereignty.

According to the official, the Argentines were so intransigent on this point that Haig found himself encountering "a repeated pattern" of arriving in Buenos Aires bearing what he thought had been an "agreed framework for negotiation" only to find the Argentines reiterating that they did not consider the sovereignty issue negotiable.

Still, the official continued, the United States did work out a proposal calling for: a cessation of hostilities; withdrawal of both British and Argentine forces from the Falklands area; establishment of an interim authority by the United States, Argentina and Britain in concert; continued British administration of the islands with Argentine participation, and establishment of negotiations for a final solution.

Britain expressed qualified willingness to explore negotiations on the basis of that plan, the official said, but Argentina, after several days of back-and-forth between Washington and Buenos Aires earlier this week, finally signaled on Thursday what Haig called "failure to accept a compromise."

"In hindsight," the official concluded, "a fair assessment must say that the United Kingdom has been reasonable and forthcoming throughout the discussions, that Argentina has been less so."

However, he added, "I don't want to suggest that the United States has broken with Argentina. It's just that we have arrived at a point in time where it is important for the U.S. public and the world community to know where we are and why."

In announcing the sanctions, Haig said, "A strictly military outcome cannot endure over time. In the end, there will have to be a negotiated outcome acceptable to the interested parties."

The senior official, saying that the United States was still hopeful that a peaceful resolution can be found, added that a "new phase is beginning" that could involve military action and said, if that happens, "it might become a benchmark for reconsideration in both capitals."