Prime Minister Margaret Thatcher told a Conservative Party gathering in Scotland tonight that the Falklands crisis had reached a point where "a negotiated settlement may prove to be unattainable" and that her government would then "have to turn to the only other course open to us."

Her pointed warning that it may soon be necessary to "throw the Argentines off" the Falklands came against a backdrop of pessimism and impatience about the negotiations at the United Nations.

Britain's U.N. negotiator, Sir Anthony Parsons, said in New York that he was returning to London to discuss the course of the negotiations. Sources said Parsons was carrying no specific peace proposals,l but U.N. Secretary General Javier Perez de Cuellar asserted that the talks had not broken down.

British Sea Harrier jet fighter again bombed positions at the airfield at Stanley on the Falklands, breaking a one-day lull in reports of hostilities, the Defence Ministry announced. No British casualties were reported.

The Argentine military command said artillery had repelled two Harriers at 6:10 a.m. EDT, Washington Post correspondent Jackson Diehl reported from Buenos Aires.

The command said in a separate communique that it had lost radio contact with the island ferry Isla de los Estados. Britain said Wednesday that one of its frigates had blown up an Argentine ship in the channel between the two main islands.

Thatcher, in a clear reference to British allies in Europe that recently have expressed concern about military escalation in the South Atlantic, said, "After six weeks we need to remind other nations that, if they believe in justice at all, they cannot be evenhanded between the aggressor and the aggrieved until the invader is thrown off the island."

The European Common Market must decide this weekend whether to continue to back Britain with economic sanctions against Argentina that are due to expire Monday. The Reagan administration is considering undisclosed requests for logistical assistance to the British naval task force in the South Atlnatic.

Thatcher and British officials were silent tonight on the Soviet Union's first official intervention in the crisis. Diplomats said the Foreign Office here would first have to study the Soviet protest today accusing Britain of unlawfully banning all ships and planes from a 200-mile-radius blockade zone around the Falklands.

Earlier, in another hard-line speech to the Conservative Party workers gathering in Perth, Scotland, Defense Secretary John Nott emphasized that Britain's task force off the Falklands was ready to enforce "a long blockade" or to secure "early repossession by force of the islands if the diplomatic efforts at the United Nations fail."

A senior Thatcher aide and other officials here said the Argentine military government still appeared to be seeking a mechanism that would assure it of eventual sovereignty over the islandss. This remains unacceptable to the Thatcher government, which wants to leave all options open.

"I hope with all my heart that the negotiations will succeed," Thatcher told her Conservative supporters. "But I would not be doing my duty if I did not warn you in the simplest and clearest terms that, for all our efforts, those of Secretary of State Alexander M. Haig and those of the secretary general of the United Nations, a negotiated settlement may prove to be unattainable. Then we should have to turn to the only other course open to us. . .."

Nott told the party workers that the task force had "cut off the Argentine garrison."

"It may be that under cover of darkness or adverse weather, some supplies are getting through by air or sea," he said. "But this is quite insignificant compared with the supplies before the blockade and the needs of the Argentine troops. The garrison know they are isolated from their friends, and they must be feeling very uncomfortable indeed when they think of the presence of the British fleet."

Nott has acknowledged that Argentine light planes apparently have continued to get through periodically, landing on grass or undamaged parts of the runway at the Stanley field.

The Defense Ministry provided no details in its announcement of the bombing raid near Stanley by the Harriers. The raid was the first military engagement reported by Britain since Wednesday.

Heavy fog and storms had curtailed air operations over and around the islands in recent days. The British forces have repeatedly bombed and shelled the air field just outside Stanley on the eastern ocast of East Falkland Island to cut off supplies to and prevent its use by the Argentines.

Brian Hanrahan of the British Broadcasting Corp., aboard the task force flagship the aircraft carrier Hermes, quoted a senoir British officer as saying, "We're in the business of attrition and blockade."

Meanwhile, British officials reported that a bomb from an Argentine Sky Hawk plane went straight through a British frigate without exploding Wednesday. It reportedly caused no casualties on the frigate, which the Defense Ministry declined to name.

From Ankara, Washington Post staff writer John M. Goshko reported the following:

U.S. officials accompanying Haig denied as incorrect a Times of London report that the United States had assured Argentina it was returning to an evenhanded approach and would not support British military actions it deems excessive or unwise.

State Department spokesman Dean Fischer, while refusing to comment directly on the Times report, said there had been "no change in U.S. policy" since the Reagan administration publicly sided with Britain.

Fischer also said the trip to Buenos Aires this week by Haig's special emissary, Vernon D. Walters, did not involve any negotiations on the Falklands and was not for the purpose of signaling a change in policy but was "only to facilitate communications with the Argentine government."