British forces reportedly captured a mountain commanding the western approach to the Falkland Islands capital of Stanley today, and the Defense Ministry announced that about 250 Argentine soldiers were killed in last week's fierce assault on Goose Green and Darwin.

Announcement of the toll, which was much higher than first thought, raised concern here that large numbers of casualties may occur in any heavy ground fighting during a British attack on the capital.

High-level Defense Ministry sources said that British troops took Mount Kent after pushing back light Argentine defense lines on the snow-dotted peak 12 miles from the capital.

Taking of the mountain puts the capital within range of the 105-mm artillery used effectively by the British invasion force in softening up Goose Green 50 miles west of Stanley last week. Military analysts expect the two-ton guns to be transported to the peaks by helicopters to add more accurate shelling to the pounding Stanley is already taking almost daily from Harrier jump jets and 4.5-inch guns on the Royal Navy task force ships.

Argentine military officials said today their ground forces and planes were slowing up the advance of British units toward Stanley, and that Argentine troops were reinforcing trench lines for a prolonged battle, Washington Post correspondent Jackson Diehl reported from Buenos Aires.

The military command reported that Argentine planes bombed British positions on Mount Kent early this morning.

Military officials reported unofficially later today that fighting had broken out between British and Argentine patrols around Bluff Cove, about 15 miles west of Stanley. Another report said British troops had advanced as far as a hilly ridge overlooking Stanley called Two Sisters.

The British Press association news agency said British troops were battling along the ridge and London's Independent Radio News claimed the British "almost certainly" had won control of the area.

Argentina is believed to have about 7,000 troops, including elite Marines, based around Stanley, and a fight to the finish with the British force of at least 5,000 could escalate the death toll sharply in the two-month-old conflict. The Defense Ministry said 125 Britons have been killed already, and Argentina is believed to have lost at least 620, not counting today's casualties.

Some Argentines were killed in the fighting for Mount Kent, while British losses were limited to fewer than 10 injured, the ministry said.

Britain had announced previously that Argentina suffered 140 injured in the Goose Green-Darwin fighting, and since wounded usually far outnumber dead it had been assumed that far fewer Argentines had been killed than the 250 announced.

The Defense Ministry said the Argentine government and the Red Cross would be notified of the identity of those killed.

One possible explanation of the severe toll came tonight from Jeremy Hands, a correspondent for Independent Television News, who said in a censored dispatch that some Argentine soldiers waved a white flag of surrender and then opened fire on approaching British troops. Maj. Chris Keeble, acting commander of the parachute battalion that took Goose Green, told Hands that at least one British officer was killed in the incident.

As a result it is believed that no white flags were trusted by the 600 paratroopers who, although outnumbered more than 2 to 1, overran enemy positions. About 1,400 Argentines were taken prisoner.

A sign that the war is becoming bitter came from other censored British reports from Goose Green tonight. Reporters said British troops found more than 9,000 gallons of napalm at a crude napalm bomb factory in the settlement. A Defense Ministry spokesman said there was no evidence that the napalm had been used.

In an indication that the British might suffer heavy casualties at Stanley, which is heavily fortified with artillery, Keeble said: "Their artillery was quite clever. They had three guns we could never ever find, and they harassed us all the way down the operation."

Defense Ministry sources cautioned that a full-scale assault on Stanley could be a few days away because of the need to move thousands of troops and tons of supplies to the front line first. However, the sources did point out that this could be disinformation aimed at Argentina.

Aides to Prime Minister Margaret Thatcher were not at all optimistic that Argentina would seek a cease-fire before a costly assault on Stanley. Britain demands that any cease-fire be accompanied by a withdrawal of Argentine troops from the islands, meaning a surrender of Stanley.

"Nothing at the moment would lead one to suggest that Argentina is willing to withdraw," a Thatcher aide said.

"The United Nations channel for withdrawal is still open," he added, referring to the virtually dormant efforts of Secretary General Javier Perez de Cuellar to mediate between Argentina and Britain.

Britain reportedly sent a message to Perez de Cuellar through its U.N. ambassador, Sir Anthony Parsons, reiterating that Argentina must withdraw from the Falklands if there is to be a cease-fire.

In Buenos Aires, the government said that two high-ranking military officers, Planning Secretary Brig. Jose Miret and Vice Adm. Benito Moya, were flying to New York to renew efforts to obtain peace through the United Nations.

Miret said he and Moya had precise instructions from the junta that would empower them to give "an almost immediate" answer to any proposal by Perez de Cuellar. "Once again we are showing that with the same determination with which we decided to go to war, we also desire peace," Miret said.

The British task force commander, Rear Adm. Sandy Woodward, is authorized to offer surrender terms to Gen. Mario Menendez, Argentina's military governor and commander on the islands, but Menendez rejected an earlier attempt.

Aside from the troops at Stanley, Argentina also has about 1,500 soldiers on West Falkland, which the British task force leapfrogged when it set up an invasion beachhead at San Carlos Bay on East Falkland 11 days ago. Military sources said they were cut off by Falkland Sound, between the two islands, and showed no sign of aggressive movement.

During the past few days Argentine Air Force attacks on the British forces have tapered off sharply, an indication that the heavy losses taken during the first week of the San Carlos beachhead have had their effect. British correspondents with the task force report in censored copy that the pilots have shifted their attacks to the ground troops instead of concentrating on the ships.

The Defense Ministry only reported one aerial incident today. It said a Sea Harrier from the aircraft carrier HMS Invincible, which Argentina claimed to have damaged heavily over the weekend, shot down a C130 Hercules supply aircraft.

Correspondent Hands described in a delayed, censored dispatch the scene as he entered Goose Green with British paratroopers and "the horrendous extent of Argentinian casualties became apparent."

"I found bodies lying in the thick gorse shrub , the wounded waiting in agony and shock, to be attended, and the frightened, beaten Argentinian soldiers moving out to be taken prisoner."

Hand said, "The officers remained aloof, looking shamed and trying to keep their men in order. Some even tried to sing the Argentine national anthem as their surrender was completed."

Meanwhile, the Defense Ministry announced that the wounded from both sides are being returned home. About 140 injured Argentines are to be transferred from the British hospital ship Uganda to the Argentine vessel Bahio Pavaiso, which was inspected by Royal Navy officers 30 miles north of the Falklands.

Another 24 Argentines and 19 Britons on the hospital ship HMS Hecla are due to arrive in Montevideo, Uruguay, Wednesday for transfer home, the ministry said.

Correspondent Diehl added the following from Buenos Aires:

Military sources said today that the ruling junta was still determined to fight a costly battle rather than surrender its garrison at Stanley to the British forces that surround it. Some Argentine Army generals were said to have told associates that they believed Argentina should avoid a heavy loss of life by agreeing to comply with U.N. Security Council Resolution 502, which calls for the withdrawal of Argentine troops from the Falklands and negotiations over Argentina's claim of sovereignty.

It was unlikely that these moderate generals would move to force a peace settlement by Argentina, however, because they feared being labeled traitors sources close to the military command said.

Military officials said today they did not expect British troops to be able in to launch a full-scale attack on Stanley until later this week. Other sources said the military command believed that Argentine forces could hold out against the British offensive for at least a week, and it was hoped that the British would be forced into a siege that might give way to a cease-fire.

"Time is passing, and it is passing in our favor," the state news agency Telam maintained. "It has been used to reinforce positions, lay mine fields, and establish lines of resistance, which will make the English advance slow, difficult and with many losses."

Argentine Military officials were quoted today as saying that British troops soon would be affected by bad weather on the Falklands. Reports said that temperatures on the islands had dropped to below freezing.

Military officials were quoted as claiming unofficially, after the British report of casualties was released, that about 600 British troops have been killed or wounded since the British beachhead was established at San Carlos on East Falkland Island.

The military also summed up its claims of British losses of planes and aircraft during the conflict, saying 25 Sea Harrier or Harrier warplanes and 22 helicopters had been destroyed and either 19 or 20 British ships sunk or damaged.

Another British Sea Harrier had been shot down over Stanley today, the military said.

In a separate development, three British journalists under arrest in Argentina since April 13 on charges of spying have had their appeal for bail refused amid strong hints in Buenos Aires that their release could only be secured by Britain in an exchange of prisoners.