British troops are moving out of their beachhead on the Falkland Islands to begin their drive to oust Argentine occupation forces, Prime Minister Margaret Thatcher told Parliament today.

A senior Defense Ministry source said the British Marines and paratroopers were advancing south, east and north from their bridgehead at San Carlos Bay on East Falkland. But he provided no details, because the Royal Marine commander of the British ground forces, Brig. Julian Thompson, is maintaining radio silence while the operation is under way.

[The British Broadcasting Corp. quoted unofficial accounts early Friday as saying that British forces met "fierce" Argentine resistance at Darwin and Goose Green 17 miles south of the British beachhead, American news agencies reported. The two settlements were believed to be the first targets of the attack.]

Helicopters ferried the British troops before dawn today to launch their assaults on hundreds of Argentine troops, according to British correspondents with confidential access to Defense Ministry officials.

There have been a series of apparently false reports in the British media that the Goose Green airstrip was recaptured earlier this week. Thatcher's statement to Parliament plus remarks late last night by Defense Secretary John Nott to Conservative Party members of Parliament that a land battle was expected "quite soon" lent credence to reports that some British troops have encountered Argentine troops near Darwin. But there have been no official reports here of contact between British and Argentine forces.

"The house would not expect me to go into details about the operations in progress," Thatcher said. Harking back to a World War II slogan, she added, "Careless talk costs lives. Too much discussion about timing and tactics could only help the enemy."

Thatcher's remarks in the House of Commons provided the first official news that the 5,000 men on the British beachhead now were taking the offensive a week after they landed at San Carlos Bay. Defense Ministry officials, speaking privately, increasingly had been questioning in recent days why little movement was being reported to London from the British forces since last Saturday, when Nott said they would move fast toward the primary target, the capital of Stanley.

The senior Defense Ministry source said that there was no single push in any one direction, but rather that the operation was one of expanding the beachhead in all directions. Some patrols already have moved forward "a long way," he added.

Thatcher also told Parliament that there were more casualties from Tuesday's destruction of two British ships in the country's worst day in the eight-week-old war. She said 33 persons are now either confirmed dead or missing and presumed dead in the sinking of the destroyer HMS Coventry and the rocketing of the merchant ship Atlantic Conveyer by one of two Exocet sea-skimming missiles fired at it.

Another 28 were injured, but the rest of the crew of the two ships, 440 sailors and civilians, were rescued. Among the eight missing from the Cunard Line's container ship Atlantic Conveyer was the captain.

For the second straight day there were no reports here of Argentine air attacks against the task force or the beachhead, leading to speculation that Argentina is having difficulty in fielding enough pilots or maintaining its jet fighters after losing more than 40, according to British claims.

A British interview with the first Argentine pilot known to be captured lent some credence to the reports. Lt. Ricardo Lucero told Glasgow Herald reporter Ian Bruce in a pooled dispatch from the task force that half of his squadron had failed to return from missions in the last six days. Argentine military authorities claimed that aircraft from his unit that failed to return had been "redeployed."

Lt. Lucero, being treated at a field hospital after ejecting from his Skyhawk jet, said: "I was shot down on my fourth attempt to attack your beachhead. Three previous attempts were shorted when your Harriers intercepted us over the sea . . . . We were told to expect the minimum in the way of antiaircraft fire over the beaches."

Meanwhile, the captain of the sunken British destroyer Sheffield said Argentina may have tried to torpedo the destroyer on May 4, Reuter reported.

Capt. James "Sam" Salt, at a press conference at a military base after returning with his ship's 260 survivors, said he had heard that two torpedoes may have missed the Sheffield by 30 feet. If true, the attack was the first naval action in the Falklands conflict by Argentine submarines, whose whereabouts have been unknown.

[Salt said a submarine may have attacked as he and his men fought a fire, caused by an Exocet missile from an Argentine warplane, which later sank the Sheffield. He said the frigate Yarmouth, one of two frigates that came alongside to help, "had to take off and attack a submarine contact. She was firing her antisubmarine weapons alongside her."]

The object of the reported offensive on Darwin and Goose Green would be threefold. Several hundred Argentine troops are there, and Britain must remove their threat to secure the southern flank before any advance from there on Stanley.

In addition, Goose Green has one of the best grass airstrips on the islands and could be used by Harrier jump jets and helicopters. Also, Darwin is astride the only all-weather track, usable by vehicles in winter, leading from the western side of the island to the capital.

The importance of Goose Green airstrip was emphasized three weeks ago, when Sea Harrier jets from the Royal Navy task force of more than 100 ships shelled it on the first day of Britain's air war to regain the islands seized by Argentina April 2. Since then it repeatedly has come under attack.

Even though the strip has been damaged, engineers can put steel tracking over the craters and make it operational in less than a day for the versatile vertical takeoff Harriers.

Use of the strip would allow the Harriers to attack the concentration of approximately 6,000 Argentine troops around Stanley from both land and sea.

Until now the air raids have been limited to sorties off the two aircraft carriers in the task force. The Royal Air Force Harriers are more effective in attacking ground installations than Sea Harriers, which are designed for air defense.

To capture Darwin and Goose Green it is likely that helicopters moved 105-mm light artillery pieces within their range of almost 10 miles to soften up the target area first.

The helicopters will play "the critical role" in the next phase of tactics now that the fighting is shifting to a land war, a senior military source said.

The task force is believed to have 40 to 50 helicopters, mainly Sea King transporters. Six of them, carrying 30 troops each and flying several sorties, can move a 600-strong battalion of soldiers to an assault location in 1 1/2 hours.

That means Brig. Thompson can ignore many of the logistical difficulties of moving his five battalions over the Falkland Islands' terrain, where peat bogs often six feet deep and soggy ground limit movement to about 6 mph over the only two tracks leading to Stanley.

Falkland Islanders normally carry elaborate winches with anchoring devices on their Land Rovers to traverse the islands, but still it is often almost as fast to walk. Land transport also has additional hazards in war, since the treeless landscape offers no cover. The best track runs from Darwin, which normally has four residents, about 50 miles along the southern coastline to Stanley.

The northern route will be virtually impossible soon, once the rains set in. It runs to Douglas settlement, then skirts Teal Inlet and crosses a valley where troops would be vulnerable between Mount Kent and Mount Estancia, where the Argentines are believed to have placed artillery.

The source said each battalion would have mounted four or five patrols since the landing at San Carlos Bay a week ago after first examining aerial reconnaissance photos from Harrier flights.

The patrols would have provided information about Argentine positions, installations and perhaps troop morale before the British troops moved out in force to confront them.

Each soldier carries a backpack weighing up to 80 pounds when transported to the battle zone but then strips down to pouches of about 26 pounds attached to his weapons belt when going into action. Each carries emergency rations for two days, a canteen of water, a ground sheet and ammunition.

The basic infantryman's weapon is a 7.62-mm self-loading, automatic rifle weighing about nine pounds. Others carry submachine or machine guns.

The heaviest hand-carried weapon used by the British forces is the Swedish Karl Gustav antitank recoilless rifle, which also can be used effectively against bunkers. The 38-pound weapon has a range of up to 600 yards and requires a crew of two. Other firepower includes 81-mm mortars with a range of almost three miles, and the 105-mm artillery.

The troops also have Milan antitank missiles.

Because of the rugged terrain, the British have brought along few vehicles, instead relying mainly on the helicopters for transportation.

The troops are also supplied with eight Scorpion and Scimitar light tanks, equipped with 76-mm or 30-mm guns.

They are wide tracked, which allows them to go through terrain where other vehicles would bog down. The tanks have a "light fingerprint," causing them to sink into mud less than a man because of their unique tracking system, according to a military officer.