United Nations Secretary General Javier Perez de Cuellar began a new round of talks yesterday in a last-ditch effort to forestall further hostilities in the South Atlantic as both Britain and Argentina continued to strengthen their military forces in apparent preparation for renewed fighting.

Perez de Cuellar met separately with British U.N. Ambassador Anthony Parsons and Argentine Deputy Foreign Minister Enrique Ros twice yesterday to discuss a U.N. proposal for a cease-fire in the conflict over the Falkland Islands and scheduled another round of talks for today after the diplomats consult with their governments.

"Progress has been achieved on some issues, while the efforts of the secretary general to bring the positions of the parties closer on the remaining points continue urgently," said U.N. spokesman Francois Giuliani last night.

For the fourth straight day there was no fighting reported in the Falklands. But Argentina said it expected at any moment either a British attack on its mainland air bases or an invasion of the islands, which Argentine forces seized from Britain April 2. The Associated Press reported there were hints from officials in London that new military action could come as soon as Monday if the U.N. talks did not succeed.

The British Defense Ministry announced that a contingent of Sea Harrier jets, refueling in mid-air and flying through the night on a 9-hour, 3,200-mile flight, had arrived on Ascension Island en route to the South Atlantic task force. The jets, believed to number 20, were expected either to fly to the Falklands armada 3,500 miles to the southwest or be carried aboard the container ship Atlantic Conveyor.

The ministry also confirmed previous reports that Ascension Island had been used for training Royal Marines and the Third Battalion Parachute Regiment for possible action in the Falklands.

Argentina reinforced its coastal defenses yesterday after Britain warned Friday that any Argentine warship or plane found more than 12 miles off the coast would be considered hostile and subject to attack. Gen. Jorge O. Garcia, commander of the Fifth Army Corps in Comodoro Rivadavia, 1,200 miles south of Buenos Aires, declared his forces "are ready to fight on any terrain and in any place."

The arrival of the British Harriers would more than double the size of Britain's air power aboard its South Atlantic fleet. The task force originally had 20 jets, but two were lost in bad weather Thursday and another was shot down by the Argentines Tuesday over Stanley, the Falklands' capital.

United Press International quoted British defense sources as saying that about 1,000 Royal Marines, sailing on the converted ocean liner Canberra, were expected to arrive in the battle zone Monday. The task force is believed to already have 1,500 Marines on board.

UPI also reported that the destroyer HMS Exeter had been diverted from the Caribbean to join the British task force and four frigates were scheduled to join the fleet as well. Britain had said previously that it was reinforcing its task force with "a number" of Nimrod reconnaissance planes refitted for aerial refueling. TOne mission of the new planes and ships is to help enforce the widened blockade that Britain declared against Argentina's 2,500-mile coastline. Previously, Britain had declared a war zone only for 200 miles around the Falklands and around British ships and planes.

Argentine Gen. Garcia said unspecified adjustments had been made in troop strength in response to the blockade and that his forces were prepared to defend their position on the mainland as well as support the estimated 9,000 Argentine troops on the Falklands.

The Argentines have claimed that several cargo ships and patrol craft have broken the British blockade, carrying supplies from the mainland to the Falklands.

"The enemy has underestimated our forces and moved its own too close without considering our reach," said Garcia.

Naval sources in Buenos Aires told Noticias Argentinas, a news agency, that the British task force would be unable to enforce the expanded blockade. "It will now have to control a maritime zone eight times larger than the one around the Malvinas the Argentine name for the Falklands ," one source said. Another source, predicting a resumption of military action, said, "Our men are ready and waiting."

Both sides appeared to be waiting to see if any progress were made this weekend at the United Nations, which diplomats from both countries and the United States say offers the last hope for a peaceful solution. But neither side appeared optimistic about the chances for success.

Argentine diplomats at the United Nations said privately they believed Britain is not sincere in its negotiating effort but is simply seeking ways to prevent an agreement without being blamed for doing so. They said they were convinced Britain wishes to strike a major military blow against Argentina before accepting a peace proposal.

The British privately made similar charges against Argentina. They said they continued to be skeptical that the Argentine military junta will live up to any commitment that its diplomats make concerning military withdrawal.

U.N. officials said they believe there is no set deadline for the talks, but that Britain may strike on Monday unless it is satisfied by tonight that there is a chance of success.

Parsons, the British U.N. ambassador, refused to set a deadline for the peace effort, saying it would be pursued "until it is clear whether it has succeeded or whether it has failed." Britain in the past has said it might conduct military action while still pursuing a diplomatic solution.

Nevertheless, Perez de Cuellar continued to remain optimistic that the talks could produce at least a temporary cease-fire accord. He said he did not agree with Argentine statements that yesterday's British declaration of a new exclusion zone around the mainland would hamper the negotiations.

"As long as there are no real hostilities, I have no reason for being disappointed," the secretary general told reporters.

Diplomatic sources have said that Perez de Cuellar's peace plan calls for a cease-fire, a withdrawal of Argentine forces from the Falklands coupled with a pullback of the British fleet, followed by some form of temporary U.N. administration of the islands while the two sides negotiate other issues. The apparent sticking points in the talks have been Britain's insistence that any cease-fire be linked to an Argentine military withdrawal and Argentina's public insistence that Britain must recognize its sovereignty claims to the Falklands before talks can begin.

But U.N. sources said Perez de Cuellar believes both sides have given some ground. He believes that Argentina, despite its public statements, has shown willingness to exclude the sovereignty issue from the present talks and that this is a major concession. At the same time, the sources said Britain has apparently agreed to drop demands that any settlement must take into consideration the wishes of the Falklands' 1,800 residents, most of whom would prefer to remain British subjects.

On the sovereignty question, Perez de Cuellar said, "This discussion of the problem of sovereignty is excluded from my exercise. I think it is for the parties to discuss at a later stage. Not now."