British forces, ending a four-day pause in the war over the Falkland Islands, shelled Argentine military positions around the Stanley airfield today, shot down a troop-carrying helicopter and attacked an Argentine fishing trawler accused of violating Britain's blockade of the islands.

The combat, reported by Britain's Defense Ministry and British correspondents with the naval task force in the South Atlantic, was the first since the British destroyer Sheffield was destroyed by a missile fired by an Argentine plane Tuesday. Attention since then had focused on diplomatic negotiations at the United Nations, which continued today.

In New York, U.N. Secretary General Javier Perez de Cuellar held talks on the Falklands late into the night after receiving a "very substantive" message from London, The Associated Press reported. Argentina said it would continue talks despite the British attacks.

The attacks regained the military offensive for Britain. Defense Secretary John Nott said in a national television interview that "intermediate steps" such as today's attacks were being taken to minimize British casualties if a full-scale invasion of the Falklands becomes necessary. He said the British task force was trying "to totally isolate the occupying forces on the Falkland Islands."

Correspondents aboard the task force flagship, the aircraft carrier Hermes, reported that British carrier-based Sea Harrier jet fighters had turned back Argentine planes apparently attempting to resupply forces on the Falklands yesterday and again today, without firing on them. They said some Argentine planes apparently had passed through the blockade last week during dense fog in which two Harriers patrolling from the Hermes were lost at sea.

The attack on the trawler brought charges from Argentine officials, which Britain denied, that after the initial attack British planes had strafed and killed survivors while they were in life rafts. The British Defense Ministry said that the crew members who had abandoned the vessel, including an Argentine naval officer, had been captured, and that the vessel was suspected of spying on the British fleet. The ministry said the vessel was also captured and that one Argentine had died, one was seriously injured and 12 had minor injuries.

The British and Argentine governments released only sketchy and conflicting information about today's combat. Initial Argentine reports of attacks by British ships, planes and helicopters heightened expectations of imminent British landings on the islands, but the British Defense Ministry spokesman said, "Our task force has not launched an invasion to recover the Falklands. There is, however, continuing task force activity to maintain the total exclusion zone around the Falklands, and to keep up pressure on the Argentine forces on the islands."

Well before dawn this morning, British ships moved in under cover of darkness close to Stanley on the eastern coast of East Falkland Island and heavily shelled Argentine military positions around the airstrip for 50 minutes.

It was "one of the most intensive yet" of several such British naval bombardments, according to one British correspondent aboard the Hermes. There were conflicting reports tonight about whether Argentine positions around the Goose Green airstrip near Darwin, about 55 miles west of Stanley, was also shelled.

During the bombardment, British Sea Harriers using the light of flares to view the action surprised an Argentine C130 transport plane escorted by Mirage fighters on their way to the Stanley field, according to British correspondents on the Hermes. They reported that the Argentine planes turned back when encountered by the Harriers.

The British correspondents said a similar incident occurred yesterday and that it appeared that the C130s were attempting either to land on grass strips that had been missed by two British bombing raids or to drop troops and supplies by parachute.

Late this afternoon, an Argentine Puma helicopter, which normally carries 20, was spotted over Stanley airfield and shot down by an antiaircraft missile fired from a British task force ship, the Defense Ministry said. No further details were immediately available.

Later this morning, at about 7:30 a.m. EDT, British officials said, a Sea Harrier from the Hermes was ordered to attack an Argentine oceangoing fishing trawler, the Narval, about 50 to 70 miles inside the 200-mile total sea-and-air-blockade zone Britain has declared around the Falklands.

Correspondents on the Hermes said the Narval had been intercepted a week ago by a task force ship on the perimeter of the blockade zone and had been warned at that time not to enter. A Defense Ministry spokesman said that the Narval had been shadowing the task force for days and that "first indications show the vessel was fitted out for surveillance tasks."

The Harrier pilot, Flt. Lt. David Morgan, said in an interview with the British Press Association correspondent aboard the Hermes that he spotted the trawler while patrolling the sea east of the Falklands.

"I radioed to ask what to do and back came the message, 'Engage,' " Morgan said. "We let off 30 mm cannon fire and hit both sides of the bridge."

A second Harrier dropped "a small bomb" along the Narval, a Defense Ministry spokesman said here. One Argentine crewman died, and about two dozen others, including an Argentine Navy lieutenant commander identified here as Gonzales Llanos, abandoned ship. They were picked up unhurt by British helicopters and taken prisoner aboard a British ship, the spokesman said.

British troops were put aboard the Narval by helicopter, the ministry said. Correspondents added that the British task force commander, Rear Adm. Sandy Woodward, will decide whether to sink the vessel or have it towed away.

The Defense Ministry denied as "ludicrous" Argentine reports that the Narval had been sunk and its crewmen strafed in lifeboats, saying it "deplored" such allegations.

As justification for the attack on the Narval, a Defense Ministry spokesman cited British warnings that any ship found within the 200-mile blockade zone around the Falklands would be regarded as hostile and liable to attack.

The second British blockade zone, which was imposed two days ago and extends to within 12 miles of the Argentine coast, only applies to military ships and aircraft, he said. He noted, however, that Friday's statement warned that "all Argentinian vessels, including merchant vessels or fishing vessels, apparently engaging in surveillance or intelligence-gathering activities against British forces in the South Atlantic, would also be regarded as hostile, and were liable to be dealt with accordingly."

By British count, Argentina has now lost its only cruiser, the General Belgrano, which sank after being torpedoed last week, a patrol boat disabled by British combat helicopters, the trawler captured today, a Canberra bomber, at least two Mirage fighters, two Puma troop-carrying helicopters and the submarine disabled when British forces recaptured the island of South Georgia, 800 miles east of the Falklands. The other Puma was reported shot down by British Marine defenders during the April 3 Argentine invasion of South Georgia.

The British have acknowledged losing the destroyer Sheffield, a Harrier shot down by Argentine antiaircraft fire over Goose Green airfield last Tuesday, the two Harriers lost in fog at sea later last week and a combat helicopter that crashed in bad weather on patrol before the task force entered combat.

The still smoldering hulk of the Sheffield was reported today to be under tow by a British ship outside the blockade zone. It was expected to be towed to port for study of damage inflicted on it by the Exocet missile fired from an Argentine Super Entendard fighter-bomber.

Asked whether there might already be some British commandos on the islands "taking some preliminary steps to prepare for a possible invasion," Defense Secretary Nott said, "I can't comment on that."

While refusing to rule out any military moves, Nott said, "There are no plans at present to bomb the Argentinian mainland" airbases.

A new public opinion survey conducted on Friday by the Opinion Research firm showed that 55 percent of the respondents now believe that "recovery of the Falklands is worth the loss of more British lives." Three out of five respondents polled by the same firm a few weeks ago had said they did not want to see the loss of even one British life.

Asked "if the Argentinian government refused to compromise and a long-term blockade seemed too risky, should the British government launch an invasion of the islands," 70 percent said yes.

"The broad overall message of this poll seems to be that the attitudes of the British people are toughening very considerably," said John Hanvey, chairman of Opinion Research, which conducted the poll for Britain's commercial television network.

The minority point of view was expressed today by about 3,000 persons who gathered in Hyde Park for a rally sponsored by the Committee for Nuclear Disarmament and addressed by left-wing opposition Labor Party politicians calling for an immediate cease-fire and the handing over of the Falklands to the United Nations. Left-wing Labor leader Tony Benn said, "We shall carry on this campaign for peace in the streets and in Parliament until the government stops the war--and we shall succeed."