The Thatcher government tonight strongly welcomed the Reagan administration's decision to "come down decisively on the side of Britain" as a major strengthening of both military and diplomatic pressures to force Argentina to withdraw from the Falkland Islands.

British Foreign Secretary Francis Pym said he would go to Washington this weekend to discuss with Secretary of State Alexander M. Haig Jr. how the United States will help Britain increase military and economic pressure on the Argentine government. He did not reveal what requests he may make for logistical military assistance offered by Haig.

"To have the world's most powerful state on our side must make Argentina see that aggression cannot pay," Pym said at a press conference here an hour after Haig's announcement in Washington of the new U.S. position. He said this constitutes "a very significant acceleration of the buildup" of military, economic and diplomatic pressures on Argentina's military government.

But Pym also warned that the failure of Haig's peace mission could make a major military conflict between Britain and Argentina more likely.

In a speech broadcast to the 1,800 Falkland Islanders, Pym said eventual further escalation of military actions beyond the blockade is "quite probable."

"I"m not all confident that war, or an escalation of some kind, may not come," he said. "That is, I'm afraid, quite probable."

Barring a challenge of the blockade by Argentine air or naval forces, however, it appeared that a major British attack on the Argentine occupation forces was not yet imminent, contrary to widespread expectations here earlier this week.

Military analysts have been careful not to talk of a time limit, but such tactics would allow some days to assess the impact of the blockade, the latest "turning of the screw" in Britain's strategy of using military pressure to get a diplomatic solution to the four-week-old crisis.

Meanwhile, the task force that reached the vicinity of the Falkland Islands earlier this week may well infiltrate marines or commandos onto remote parts of the islands, where there are no Argentine forces, to prepare beachheads for an eventual invasion. Although the Ministry of Defense has put a ban on all operational information, it is widely believed that some specially trained marines are already on the islands.

Aside from diplomatic considerations, the slow escalation of military pressure also fits into the requirements of the task force which now contains about 70 ships strung out over 8,000 miles of the Atlantic Ocean.

Military analysts say the task force would not be ready for about three weeks to launch full-scale hostilities, including a frontal attack on Port Stanley where most of the Argentine forces are believed to be based.

For such an assault, if it becomes necessary, Britain would need more air power and ground forces than it now has around the Falklands and the island of South Georgia, 800 miles east of the Falklands, which British forces recaptured last weekend in a virtually bloodless operation.

Argentina has at least numerical air superiority over the British task force which has only 20 Sea Harrier vertical takeoff jet fighters and 37 antisubmarine helicopters. Argentina has more than 100 U.S.-built Skyhawks and French- and Israeli-built Mirages.

Twenty more Harriers are on board a container ship that left Britain last weekend but it will take two weeks or more to get them to the battlefront and into position on warships.

In his remarks at the news conference, Pym said, "We shall not abandon our efforts for a peaceful settlement, but Argentina must withdraw, as the U.N. Security Council resolution demanded four weeks ago."

He said he would also discuss with Haig and with U.N. Secretary General Javier Perez de Cuellar in New York possible new channels for negotiations with Argentina if it eventually decides to withdraw its occupation forces from the Falklands. Britain would consider an international conference "or any other kind of negotiation," Pym said.

Pym and British diplomats dismissed today's statement by Argentine Foreign Minister Nicanor Costa Mendez that his government was ready to comply with the U.N. Security Council resolution calling for an Argentine withdrawal but that its claim to sovereignty over the Falklands was still not negotiable.

Britain is now itself turning to the United Nations for possible diplomatic assistance, despite Thatcher's earlier doubts that little more could be accomplished there beyond the Security Council resolution calling for an Argentine withdrawal.

"Our minds are not closed to U.N. involvement in either a new diplomatic mechanism or an interim administration of the islands after an Argentina withdrawal," said one British source.

This was being done, sources here said, to indicate the Thatcher government's willingness to try to keep diplomatic channels open under pressure from Parliament and Britain's European allies to exhaust all possibilities for a peaceful settlement before military escalation into a bloody conflict.

British diplomats have closely studied a wide range of negotiating "mechanisms," including a conference chaired by a neutral party in which Britain and Argentina could bargain with each other, but which Thatcher's government would consider only after a firm Argentine commitment to unconditional withdrawal from the Falklands, sources said. Pym again ruled out today direct negotiations with Argentina before any withdrawal.

Both the West German and Italian governments, which have joined other European Common Market members in supporting Britain with economic sanctions against Argentina, publicly stated their concern today about military escalation in the Falklands crisis. A government spokesman in Bonn said West German Chancellor Helmut Schmidt had discussed this with Thatcher in a telephone conversation yesterday.

The Italian Foreign Ministry said in a statement today that "the undeniable need to restore a position of right . . . must not imply the unleashing of armed conflict, the consequences of which would have repercussions far beyond the area of interest of the two countries concerned."

"Conscious of its particularly delicate position in this affair as a result of its deep ties with the people of Argentina, Italy launches a heartfelt appeal to both sides for the greatest possible moderation at this crucial moment," the statement added. A large number of Argentines are of Italian descent.

Newspaper editorials in West Germany, Italy, France and other European countries also have cautioned Britain that an escalation of the conflict could risk the loss of its international support in the dispute.

Recent parliamentary debates here also have produced growing demands that the Thatcher government try every possible diplomatic and military tactic to secure an Argentine withdrawal before resorting to a frontal assault on the occupation forces on the Falklands that could produce heavy casualties on both sides and escalate into wider conflict. Although Parliament does not yet appear to be seriously split over this, a senior member of Thatcher's Conservative Party today described the parliamentary and public opinion consensus behind Thatcher's strategy as "fragile" and unlikely to survive much bloodshed in the South Atlantic.

Thatcher herself, who chaired two meetings of her "war Cabinet" of senior ministers today to discuss the new developments in the crisis, avoided public comment on them. She left 10 Downing St. tonight for the prime minister's country home at Chequers without speaking to reporters.

A Downing Street spokesman said Thatcher intended to stay at Chequers during a long May Day holiday weekend, but would return to London at an hour's notice if necessary for more crisis meetings.

Although the Ministry of Defense will provide no information, it is believed that there are only about 1,500 British marines and paratroopers in the immediate combat area now.

Residents of Ascension Island, about 3,500 miles north of the Falklands in the mid-Atlantic, have reported that the converted liner Canberra, which is carrying 2,000 troops, has remained in the vicinity of the island. British correspondents aboard the Canberra said today in censored dispatches that the troops are practising assault maneuvers.

At least another 1,000 troops just set sail for the Falklands this week. So it is estimated that it will take at least another two weeks to get all these forces into the battle theater for possible operations.

Even then Britain would have only a maximum of 5,000 troops available, perhaps as little as half of the number of Argentine troops. Military analysts say that a 3-to-1 numerical advantage is usually recommended for a frontal attack on an entrenched enemy to succeed.