The Foreign Ministry declared tonight that Argentina had informed the United Nations of its willingness to "negotiate a peaceful solution" to the Falklands crisis, adding that "the first move . . . should be an immediate cease-fire."

Just before tonight's announcement, Foreign Minister Nicanor Costa Mendez met with the ruling military junta. Earlier in the day, ministry officials had discouraged any speculation that Argentina would accept a cease-fire.

Costa Mendez said then that the proposal from U.N. Secretary General Javier Perez de Cuellar, which is understood to include a cease-fire call, had been subjected to "profound analysis." But he had added that, for U.N. mediation efforts to be effective, "they will have to adjust to our propositions." Prior statements here have stressed that Argentine sovereignty over the Falklands be accepted from the start. Tonight's brief statement did not raise that issue.

In his note to Perez, Costa Mendez indicated he would discuss Argentina's response to particular ideas from Perez in a personal meeting in New York and that he would go there should a Security Council meeting be called on the issue.

In New York, Perez said he had received a "positive reaction from the Argentine government" to his proposals.

Shortly before tonight's Foreign Ministry statement was issued, spokesman Hernan Massini Ezcurra said that "because of British operations, the meeting of the Security Council has been blocked . . . and therefore, for the moment, Dr. Costa Mendez will not travel" to New York.

Tonight's call by Argentina for an immediate cease-fire came only hours after Foreign Ministry sources disparaged any such initiative. One of them said yesterday that should such a proposal be put before the Security Council, the Soviet Union would be likely to veto it in behalf of Argentina.

The Foreign Ministry sources had said there was little feeling here of substantial movement in the diplomatic process.

One government source said the apparent British move away from military action and toward diplomacy was regarded with suspicion in some quearters here. "It could be they are trying to buy time until their reinforcements arrive," the source said.

On the war front, casualties continued to rivet public attention. As the first survivors from the sunken Argentine cruiser General Belgrano arrived to a heroes' welcome on the mainland today, it became increasingly clear that the country has suffered heavy casualties during four days of naval combat with Britain.

With fighting in the South Atlantic in an apparent pause, the military command reported finding no new survivors from the cruiser. A total of 680 crew members were reported to have been rescued yesterday.

The chief of intelligence for naval operations, Capt. Juan Carlos Bou, said the rescue operation was continuing with the hope of finding more of the 52 lifeboats that were dropped from the Belgrano Monday. He noted, however, that the boats have been in the water nearly 48 hours, and other reports by Argentine journalists in Ushuaia said temperatures had dropped below freezing in the rescue area and that waves were over 25 feet.

The cruiser was said by the Foreign Ministry to have had a crew of 1,042 when it was torpedoed Sunday by a British nuclear submarine.

With no clear end in sight to the conflict here, Argentine Economics Minister Roberto Alemann announced a broad series of emergency measures meant to rescue the crippled economy, including a 17 percent devaluation of the peso and a 30 percent increase in gasoline prices.

The military command reported late this afternoon that there had been no contact between British and Argentine forces since 12 p.m. yesterday EDT, when Britain bombed the airfield at Darwin on the Falklands. No further details were offered today on the Argentine air attack on the British destroyer Sheffield yesterday morning.

The military command reported that the Argentine tug Sobral, which was involved in a battle with British Sea Lynx helicopters early Monday morning, has successfully sailed into the southern port of Puerto Deseado. The military communique said, however, that the World War II-era tug had been badly damaged and that some of its crew had been killed.

Military officials offered no estimate of the total number of sailors killed on the Belgrano and Sobral by late today, and gave no information on the fate of several Argentine pilots whose planes have been reported shot down. There have also been no casualty reports from the Argentine forces on the Falklands, where British forces have launched at least four bombing raids, and several other helicopter attacks by Argentine accounts.

Argentine newspapers reported today that between 800 and 850 of the Belgrano's crew could eventually be saved. A television report also said that two pilots of a Canberra bomber shot down on Saturday had been picked up by an naval vessel. These reports were not confirmed by military officials, who have not yet acknowledged that a Canberra bomber was shot down.

Even with the most optimistic reports, however, Argentine losses appeared today to be the greatest since September 1866, when Argentina suffered 1,500 casualties during a battle in its war with Brazil and Uruguay against Paraguay.

Attention focused during much of the day on the dramatic arrival in the southern port city of Ushuaia and the Navy air base at Bahia Blanca south of Buenos Aires of the first survivors of the Belgrano.

A boat carrying 400 crew members of the Belgrano was reported to have arrived in Ushuaia this morning from the frigid waters off the southern tip of South America where the boat sank Monday afternoon. At noon, a planeload of 80 crew members, including the ship's captain and second in command, landed in the Navy air base of Comandante Espora in Bahia Blanca.

The new economic package announced today seemed to be meant to prepare Argentina's economy for a long-term crisis. The measures also appeared to quiet growing calls by political parties and labor unions for a reorganization of the military government's conservative economic policies.

Alemann said in a television address tonight that he expected the emergency package "to satisfy a legitimate concern of Argentine industry, pressed by a long recession and oppressed by the burden of high interest rates."

In addition to the devaluation of the peso, other measures included the granting of a new subsidy for key exporters and the lowering of tariffs for some industrial imports. Imports of items deemed "highly dispensable" were ordered curtailed for 45 days.

Officials also ordered a 30 percent increase in the price of gasoline--to about $1.60per gallon--and reduced the guarantee on bank deposits to 80 percent from 90 percent for sums above $10,000. That action was aimed at stemming outlays because of the collapse of financial institutions. Four more closed today.

The overall measures are aimed at shoring up Argentina's sagging reserves of foreign currency and boosting its exports at a time when many of Argentina's traditional markets have been cut off by the economic sanctions taken by the European Community and other allies of Britain, including the United States.

During the month of April, the central bank's dollar reserves fell by $500 million, including a plunge of $162 million in the last week, according to government figures released yesterday.

While devaluing the peso to 14,000 to the dollar to make Argentine exports of such products as cereal, meat, and fruit more attractive to foreign markets, the government also increased the tax grain exporters must pay on the new incoming dollars by 7.1 percent.

Officials said the new tax would be used to help defray the cost of Argentina's military operations, which had been estimated at $500,000 a day even before fighting began.

Among the imports subject to prior approval by the government for the next 45 days are auto parts, alcoholic beverages, perfume, clothing, meat, fruit, fish, and textile, plastic, chemical and leather products, according to a report in the financial paper Ambito Financiero.

It is hoped that the ban on these imports and lowered customs barriers for other items will conserve dollars and stimulate industry mired in one of the country's first recessions.

The military communiques issued today left a number of points unexplained from the four days of fighting in the South Atlantic since last Saturday. While still not confirming officially that Argentine planes had destroyed the Sheffield, military officials also did not account for pilots of Argentine jets that have been shot down in addition to those on the Canberra.