Britain declared a military blockade against Argentina tonight, announcing that any Argentine warship or plane found more than 12 miles from the country's coast would be regarded as hostile and subject to attack.

Foreign Secretary Francis Pym also warned that Britain might bomb air bases on the Argentine mainland and conduct new raids on airfields in the Falkland Islands if Argentina attempted to attack British forces or resupply its troops in the Falklands by air.

The British declaration appeared to be a prelude to a renewal of military activities in the South Atlantic. It also seemed to mark an end to the three-day lull that both countries have maintained while diplomats from the United States, Peru and the United Nations attempted to work out a peaceful solution to the Falklands conflict.

The Argentine Foreign Ministry declared, "This extension of the area of hostilities constitutes another escalation of the conflict attributable, like the others, solely to the decision and responsibility of the United Kingdom," Washington Post correspondent Jackson Diehl reported from Buenos Aires. The communique said Britain was "creating situations that seriously disturb all possibility of negotiating."

The diplomatic effort appeared stalled again today, despite a last-ditch attempt by U.N. Secretary General Javier Perez de Cuellar to head off renewed hostilities. London continued to insist that any cease-fire agreement include an Argentine commitment to withdraw its forces from the Falklands, while Buenos Aires said it would accept no cease-fire without recognition of Argentina's claim to sovereignty over the islands.

Argentine Deputy Foreign Minister Enrique Ros arrived in New York Friday to discuss the U.N. cease-fire plan, The Associated Press reported. His arrival came as Argentina protested the new British blockade to the Security Council.

The new blockade, which would attempt to quarantine Argentina's 1,500-mile coastline, greatly expands previous exclusion zones established by Britain around the Falklands.

The move, announced by the Ministry of Defense, is designed to give Britain's South Atlantic task force the maximum opportunity to shoot down Argentine attackers before they can reach ships and stage a repeat of Tuesday's attack on the British destroyer Sheffield. Twenty British seamen were reported killed when the warship was crippled by a missile fired from an Argentine fighter-bomber.

To enforce the expanded blockade, the Defense Ministry announced that "a significant number" of Sea Harrier vertical-takeoff jets "have been deployed with air-to-air refueling to the South Atlantic."

Ian McDonald, the Defense Ministry spokesman, also said Nimrod reconnaissance planes have been refitted for aerial refueling and "will shortly be deployed to the South Atlantic." It is expected that the planes will carry out reconnaissance flights from British-owned Ascension Island in the mid-Atlantic, about 3,500 miles from the Falklands.

Military analysts said Britain's chief problem in enforcing the new blockade will be Argentina's Air Force of more than 100 jets. To make the blockade stick, warships from the task force will have to move within range of the planes, a tactic that could set off a new round of fighting.

Another problem is the Argentine Navy, which has not been involved in any known offensive action since the start of hostilities. It is not known here whether Argentine warships are within the 12-mile limit.

Today's action appeared designed in part to assuage right-wing Conservatives who have been demanding since the destruction of the Sheffield that Britain bomb Argentine air bases. Such a move would be seen as a major escalation of hostilities and could cause Britain serious diplomatic problems.

The imposition last week by Britain of a 200-mile exclusion zone around the Falklands led directly to four days of sea and air warfare capped by the sinking of the Argentine cruiser General Belgrano, with the estimated loss of as many as 360 Argentine seamen, and the destruction of the Sheffield.

McDonald said the new blockade was necessary "because of the proximity of Argentine bases and the distances that hostile forces can cover, particularly at night and in bad weather."

Aside from the deployment of the Harriers and Nimrods, there are reports that Britain plans to reinforce the task force with more frigates and destroyers, two of which are equipped with new antimissile missiles that could have defended against the weapon that destroyed the Sheffield.

Foreign Secretary Pym again blamed "Argentine intransigence" for scuttling a U.S.-Peruvian peace proposal that he said was very similar to that being pressed by Perez de Cuellar. "It is difficult to believe that Argentina, having rejected ideas devised by U.S. Secretary of State Alexander M. Haig and the president of Peru, can now accept the secretary general's ideas, which have such a similar basis," he said.

Pym told a press conference that if the Argentines repair the bombed airfield at Stanley, capital of the Falklands, Britain "would bomb it again." Asked about the possibility of British bombing raids against mainland air bases, he said, "I would not deny that. We do not want an escalation of military activity, but I would not exclude any possibility whatever."

The Times of London said Prime Minister Margaret Thatcher had secured "the authority of the full Cabinet for an attack on air bases in mainland Argentina" if necessary to protect the naval task force.

The Associated Press reported from the United Nations:

Argentine Deputy Foreign Minister Ros, arriving in New York, held an hour-long closed-door session with Secretary General Perez de Cuellar. He emerged saying his comments on the U.N. peace proposal "were carefully registered" by Perez de Cuellar and that the two men would meet again Saturday.

But Argentina warned in advance of Ros's arrival that it would not accept any peace proposal that did not include recognition of Argentine sovereignty over the Falklands, which it seized from the British April 2. Diplomatic sources have said the U.N. plan leaves the sovereignty issue as a subject for later negotiation.

Perez de Cuellar, who Thursday said both sides had given him a "positive response" to his proposal, refused to be pessimistic about today's developments. "We are moving; we are moving," he told reporters.