The British government, responding to mounting domestic and international pressure, declared today it was actively supporting a series of new diplomatic efforts aimed at negotiating a peaceful settlement of the Falkland Islands crisis.

In what appeared to be an abrupt change in tone from the tough military rhetoric of recent weeks, Foreign Secretary Francis Pym told Parliament his government was "working very actively" on new proposals offered separately by U.S. Secretary of State Alexander M. Haig Jr., Peruvian President Fernando Belaunde Terry and United Nations Secretary General Javier Perez de Cuellar.

Pym also said for the first time that a U.N. trusteeship of the disputed Falklands "might very well prove a highly suitable" long-term solution, if a cease-fire and Argentine military withdrawal from the islands could first be negotiated.

"The military losses which have now occurred on both sides in this unhappy conflict emphasize all the more the urgent need to find a diplomatic solution," Pym told Parliament in announcing the new diplomatic moves.

Perez announced Wednesday night that he had received "a positive response from the Argentine government" to his peace proposal, Washington Post special correspondent Michael J. Berlin reported from the United Nations. The secretary general said he expected a British reaction Thursday. A British spokesman in London had no comment on the Perez statement.

France and West Germany, which until this week had stood united behind London in the Falklands crisis, formally urged an immediate end to hostilities in statements approved by their cabinets Wednesday, Reuter reported.

Pym's comments referred to the sinking of the Argentine cruiser General Belgrano by a British submarine and the destruction of the British destroyer Sheffield. As many as 400 Argentine seamen may have perished with the Belgrano after it was torpedoed Sunday. About 30 of the crew of the Sheffield are missing and feared dead and the government today reported that as many as 60 more were injured after the destroyer was hit yesterday by a missile fired by an Argentine warplane.

Just a few days ago, officials here were emphasizing their plan "to put the screws on the Argentines" through a series of military actions aimed at driving Argentina's forces from the Falklands, and these officials still insist that an Argentine withdrawal is essential to any diplomatic solution. But the loss of the Sheffield and a British warplane--which were the first deaths Britain has suffered in the conflict--has sharply changed the public rhetoric and emphasis of the British effort.

Pym said he sent "a constructive contribution of our own" to Haig last night and had consulted with the secretary of state again today on the formation of new peace proposals to be transmitted to the military government of Argentina.

Pym said he also is keeping in touch with Perez about similar "helpful ideas" that "seem certain to be reflected in the basis of any solution we are able to achieve."

While warning "it is too early to be optimistic," a senior British source said "Haig is back in business." He described the proposals being considered by Prime Minister Margaret Thatcher's government as "ideas of Peruvian origin with Al Haig's points added."

An aide to Thatcher said her government devoted "a lot of diplomatic effort" during the past two days to seeking a negotiated settlement. While this effort continues, the aide said, Britain was willing to limit military action for the moment to enforcement of its 200-mile blockade zone around the Falklands "if that is not challenged."

Ministry of Defense officials said they were not aware of any new military action today following yesterday's destruction of the Sheffield and British air raids on Falklands airstrips.

Defense Minister John Nott told Parliament that he had "no knowledge" of American intelligence reports that a major naval battle was under way in the South Atlantic. When that answer brought obvious murmurs of dissatisfaction from some opposition Labor Party members, Nott continued: "I can't be sure . . . but we have no reports on it and I checked on it quite recently."

The new settlement proposals appear to be variations combining a call for a cease-fire by both sides with Haig's earlier proposals for a phased Argentine withdrawal from the Falklands and a pullback of the British naval task force. This would be followed by a tripartite interim administration by Britain, Argentina and the United States using British civil servants while the islands' long-term future is negotiated.

But Pym and others here said that Britain would have to be assured of Argentina's commitment to withdraw before it could agree to a cease-fire. Otherwise, said Stanley Clinton-Davis, an opposition Labor Party Parliamentary spokesman on foreign affairs, Argentina could use a cease-fire and relaxation of the British blockade "as a breathing space simply to reinforce their garrison on the Falklands."

It was not known how the new settlement ideas deal with the primary sticking points of earlier negotiations: Argentina's insistence on a guarantee of eventual sovereignty over the Falklands and British insistence on self-determination for the 1,800 islanders, most of whom want to remain part of Britain. Pym has acknowledged that Britain is considering the possibility of a negotiating conference of British and Argentine representatives, chaired by another nation. It is possible that the islanders could also be represented at such a conference.

Pym's diplomatic efforts appeared to receive wide support in a hushed and somber House of Commons that also heard Defense Minister Nott report on yesterday's attack on the Sheffield. A majority of members of Parliament appeared to support an increased emphasis on negotiations without weakening Britain's military position or its demand for an Argentine withdrawal.

While insisting that any British agreement to a cease-fire be dependent on an assured Argentine withdrawal, Deputy Labor leader Denis Healey said, "I think we all feel if military escalation continues as it has in the past few days there will be more casualties than there are inhabitants of the islands."

Nott devoted as much emphasis to diplomacy as to the military situation in his address today. "We want to attain a diplomatic settlement as soon as possible," he said.

Nott gave few details of the Sheffield's destruction, simply saying the destroyer was 70 miles from the Falklands when it was hit amidships yesterday afternoon by one of two French-built Exocet missiles fired from a Super Etendard jet fighter, also made in France. The explosion caused a fire that spread out of control. Nearly four hours later the ship was abandoned.

Survivors from the 270-man crew were picked up by other British ships. It is presumed that any still missing would have died by now in the icy waters of the South Atlantic, where waves are as high as 30 feet.

Nott did not say whether the Super Etendard, which flies at the speed of sound, took off from the Argentine mainland or from the country's lone aircraft carrier. Most analysts believe that the plane, which has refueling capacity, attacked from the mainland about 300 miles west of the islands.

When fired from high altitude, the Exocet has a range of about 28 miles and travels at almost the speed of sound, skimming just above the water to avoid radar detection.

The Sheffield was an escort ship for the Hermes and the Invincible, Britain's two aircraft carriers in its task force of more than 70 ships.

Officials at the Ministry of Defense said the ship was still afloat and burning. It was reported that some officers returned to the ship to examine the damage more closely. The replacement value of the ship is estimated at about $220 million.

Washington Post foreign correspondent Jay Ross contributed to this report.