British forces speeded up their methodical advance across East Falkland Island late last week in a secret, three-day airlift of troops to the Fitzroy and Bluff Cove settlements about 15 miles southwest of Stanley after their commander was told in a call from a pay telephone that Argentine forces had left the area undefended.

Discovery of the new British salient by Argentine forces early this week, after a break in the heavy cloud cover over the Falklands, led to Tuesday's Argentine air attacks on British warships bringing reinforcements and supplies to the soldiers of Britain's Fifth Infantry Brigade of Scots and Welsh Guards.

The ships were attacked while unloading supplies for the British troops at Fitzroy and Bluff Cove, according to the ministry. If their progress is not stopped by the Argentine air strikes, these soldiers are likely to link up just outside Stanley, the Falklands capital, with other Fifth Brigade infantrymen and Royal Marine commandos who have been moving by land and helicopter through northern and central East Falkland to the British front lines on high ground around Mount Kent, seven miles from Stanley.

Some British commandos already have advanced farther forward to within a few miles of Stanley, capturing mostly abandoned Argentine outposts on foothills just west of the town, according to British correspondents on the Falklands. Others have been airlifted by helicopter with artillery to high ground just across a small bay from Stanley, north of its airport, putting it in British land artillery range for the first time.

But British advance forces had not yet moved this close to Stanley last week, when Fitzroy and Bluff Cove were reoccupied by Fifth Brigade troops who had landed only days before at the San Carlos beachhead on the northwestern corner of East Falkland after being transferred to landing ships from the luxury liner Queen Elizabeth 2 off the island of South Georgia, 800 miles east of the Falklands.

They were expected to make their way east much more slowly over a dirt track from Darwin and Goose Green, south of San Carlos, after the outposts were recaptured by British paratroopers. Military engineers were working to make the track usable for heavy vehicles, which could have taken a number of days.

But when the Fifth Brigade's commanding officer, Brig. Tony Wilson, got as far east as Swan Inlet, about halfway between Darwin and Fitzroy, he was told by an advance British patrol that the telephone lines were still working, according to dispatches from British correspondents on the Falklands. Wilson inserted a 50-pence coin (about 90 cents) into a pay phone and dialed Falkland Islander Reg Binney in Bluff Cove, according to Britain's Press Association.

"Are the Argies still at Fitzroy?" Wilson was quoted as asking Binney. "No," he reportedly replied. "Why don't you come over and join us?"

So, in a departure from the invasion plan, Wilson ordered that Fifth Brigade troops be rushed to Fitzroy and Bluff Cove by helicopter from the San Carlos beachhead, their movement hidden from Argentine observation by the heavy cloud cover then over the Falklands. Supplies were sent to them from Goose Green on a recaptured former Falkland Islands Co. supply ship.

The supply ship Monsunen was attacked by a British warship in late May and abandoned by its Argentine crew, which had been using it to ferry supplies to Argentine forces on West Falkland Island. The 19-year-old daughter of a Falkland Islands Co. manager at Goose Green, Jane Harcastle, dived into the icy water last week to untie a rope the Argentines had left around the ship's propeller, according to delayed British dispatches from the Falklands released here early this morning.

These dispatches, sent to London from the Falklands on Saturday, were blocked here by military censors until yesterday's Argentine air attack made known the position of the British forces at Fitzroy and Bluff Cove. Television correspondent Michael Nicholson had succeeded in getting past the censors only a tantalizing reference to operations of "extraordinary daring" that he hoped to reveal later. These apparently were the capture of heights north of Stanley Airport and the secret advance to Fitzroy and Bluff Cove.

During the next few days, according to the Press Association, more troops were flown from Goose Green to Lively Island, off East Falkland south of Swan Inlet, from where they were taken to Fitzroy and Bluff Cove by landing ship. Equipment and supplies for the troops were being landed by the ships yesterday when clearing weather enabled the Argentines to detect the British operation and call in air strikes.

But by this time, mobile Rapier antiaircraft missile systems had been landed and installed around Bluff Cove, according to the Press Association. These missiles apparently hit the attacking Argentine planes the British Defense Ministry said were shot down and damaged.

The chairman of Thatcher's Conservative Party and a member of her "war cabinet," Cecil Parkinson, warned yesterday of "very tough days ahead," but predicted that "an aggressor is going to be expelled from British territory."

Thatcher repeated in Parliament that "if the Argentines tell us they are prepared to withdraw we shall enable them to do so with safety, dignity and dispatch." But, she said, "so far we have had no positive response" to calls from herself and the British commander on the Falklands for the Argentines to surrender.

In a British television interview last night, U.S. Secretary of State Alexander M. Haig Jr. called on Argentina to withdraw its troops from the Falklands now that the British held the upper hand in the war. "I would hope the leadership in Buenos Aires would have the good sense to prevent further bloodshed and recognize the untenable position they now find themselves in on the islands," Haig said.

Haig told a press conference here yesterday that the U.S. response to Britain's request for participation in a multinational peace-keeping force on the Falklands after their recapture "would depend on its mandate, on the tenure, and on the political framework in which it is put forward."

This appeared to confirm doubts expressed privately by U.S. officials about contributing to a force protecting a British-ruled or independent Falklands from Argentina, rather than helping to achieve a negotiated settlement between Britain and Argentina on the islands' future.

Saying the Reagan administration has yet to make a judgment about British and Argentine sovereignty claims, Haig said, "What we have claimed is Argentina has no right to invade the Falklands with armed force, and we are opposed to that action and we have supported Britain's contention of that action."