With the British fleet apparently closing in on the Falkland Islands, Argentina today turned down a request from Secretary of State Alexander M. Haig Jr. to come to Buenos Aires with new proposals for a negotiated settlement to the Argentine-British confrontation.

Echoing British assertions in London that an attack might be imminent, diplomatic sources here said that the fleet would be within striking distance of the Falklands by Thursday.

A full-scale invasion of the Falklands, according to these sources, is improbable, but they expect the British to "chip away" at Argentine military forces with the likely intention of inflicting setbacks that would weaken the position of the ruling military junta here and possibly cause it to fall.

Against the backdrop of possible new confrontation, Argentine sources today confirmed that the military government had turned down a proposal made by Haig to come to Buenos Aires this week.

Widespread reports here added that the Argentine government had responded to the Haig proposal by saying he should submit any new U.S. plan for a diplomatic settlement to Argentine foreign minister Nicanor Costa Mendez, who is in Washington. That is the procedure Haig followed in dealing with British Foreign Minister Francis Pym last week.

The Buenos Aires daily, Conviccion, said to have close ties to the Navy, described Haig today as "the ex-mediator." In a speech at the Organization of American States yesterday, Haig implicitly criticized the Argentine position, saying the OAS' Rio Treaty was not appropriate for the crisis. While Haig said the treaty is designed to resist use of force from outside the Western Hemisphere, he added without naming Argentina that in this case force had already been used.

In Washington, U.S. sources said that while Haig's proposed third shuttle to Buenos Aires had been turned down for now, their impression was that the Argentines did not preclude his making it later.

Argentina's president, Gen. Leopoldo Galtieri, met with the Air Force and Navy chief that with him form the ruling junta. He then called an emergency meeting of the Cabinet. This activity tonight followed an earlier round of meetings by high officials at the Foreign Ministry, who were said by government sources to be drawing up a lengthy document for the junta's consideration.

The apparent Argentine rebuff of Haig came as foreign analysts here said time is becoming a crucial factor for British military operations. Within two weeks, weather conditions in the South Atlantic are expected to make operations hazardous and difficult. Gale-force winds, mammoth seas, snow storms and sub-freezing temperatures are anticipated.

With these conditions limiting British options, these analysts expect an effort by the British to engage an Argentine warship or to provoke an aerial encounter out of range of Argentina's land bases. Another source, however, insists that a troop landing on West Falkland Island--possibly commando raids--is another possibility, as a follow-up to the successful occupation of South Georgia Island on Sunday.

A full-scale assault on the Falklands is regarded as potentially "suicidal" by foreign analysts. An all-out blockade of Argentine ports is also regarded as impractical. It would affect countries that ship through Argentine ports and would involve potential confrontation with merchant vessels from throughout the world, including the Soviet Union.

As tension continued to grow the Interior Ministry issued a statement urging citizens to avoid "procedures against the general interest."

The statement, read by an Interior Ministry official at a luncheon meeting, said: "The moment of natural euphoria having passed" in Argentina, "it is necessary to return to the time of work and effort."

The statement called for "a patriotic attitude of serenity and firmness," and asked Argentines in particular not to hoard food, medicine and fuel and to stop withdrawing their deposits from banks.

The country now expects "work and loyal collaboration by workers" and "grandeur and solidarity from the political leadership," the statement said.

While calling for sacrifices from citizens, the government also gradually stepped up internal security measures today.

In a series of decrees issued late last night and today, the military specified that persons seeking to avoid military draft orders could be punished with the death penalty, and ordered a series of manufactured items to be subject to military approval for export. The items included arms, chemicals, lead products, binoculars and topographical and meteorological equipment.

Military authorities banned foreign journalists today from southern port cities where naval and air bases are located. The measure was taken, according to an Army statement, to "avoid the broadcast of news of an operational character outside the country."

The correspondents were ordered to return to Buenos Aires within 48 hours. Civil defense exercises, including practice blackouts, were reported under way in some of the southern port cities. Three British journalists were detained last week and are being held on espionage charges stemming from their allegedly having photographed military installation.

The American Embassy said it has not removed any of its personnel to other countries but certain classified materials have been sent out of Argentina.

Reports continued, meanwhile, that Argentine naval commando forces on the island of South Georgia were continuing irregular operations against British troops. The reports, which military officials refuse to publicly confirm, said that Argentine forces had downed a British helicopter and damaged several landing craft.

The private news service Noticias Argentinas quoted military sources as saying both Argentine and British forces had received casualties during the continued fighting. The agency said the Argentine naval resistance was being carried out by the "Lizard" group, a special strike force dating from the military government's campaign against internal opponents during the 1970s.

In the limited military encounters between Britain and Argentina thus far, it is Argentine naval forces that have suffered most. They took an undetermined number of casualties in their occupation of the Falkland Islands on April 2. Their troops on South Georgia were taken prisoner after only minimal resistance, according to claims from the British government, and they lost a World War II vintage submarine.

These setbacks, some diplomats here believe, have damaged the Navy's prestige, especially since the Navy reportedly was the strongest advocate of the Falklands' occupation.