Britain today announced a total air and sea blockade of the Falkland Islands beginning Friday morning and threatened to attack any aircraft or ship within a 200-mile radius of the islands.

The blockade, to be imposed by the British naval task force now in the vicinity of the Falklands, was described by officials here as "another turning of the screw" to persuade Argentina to withdraw its troops before Britain tries to remove them by force.

Government spokesmen said Britain was considering formal proposals from the United States to settle the crisis. But they said no British response was likely until Argentina reacts to the proposals, and they expressed no optimism that the American plan would be accepted by the Argentine military government.

Argentina said Wednesday that a new U.S. proposal was "under study," but government officials indicated that the plan did not satisfy the Argentine military command, Washington Post correspondent Jackson Diehl reported from Buenos Aires.

While providing no details of the proposal, which was described as being endorsed by President Reagan, government officials indicated that it did not meet Argentina's demand for assurances of eventual sovereignty over the South Atlantic islands.

The Argentine military junta said it expected British military action within 48 hours, and government sources said Argentina did not intend to offer new formulas of its own for an accord with Britain.

The new Falklands blockade, announced after an emergency meeting of Prime Minister Margaret Thatcher's Cabinet, is seen here as a prelude to a British attempt to recapture the Falklands from Argentine forces who seized them nearly four weeks ago. A senior British source said the government was not slowing its timetable for military action while Argentina considered the American peace proposals.

Using its nuclear submarines, Britain established a 200-mile maritime exclusion zone for Argentine ships around the Falklands April 12. But the blockade announced today will be far more sweeping because it includes aircraft, including those on the ground in the islands, and applies to aircraft and ships of all nations.

"Any ship and any aircraft, whether military or civil, which is found within this zone without due authority from the Ministry of Defense in London will be regarded as operating in support of the illegal occupation and will therefore be regarded as hostile and will be liable to be attacked by British forces," the announcement said. The blockade is to take effect Friday at 7 a.m. EDT.

Demanding that Argentina remove warplanes and transport aircraft it has put on the islands since the invasion, Britain announced, "Any aircraft on the ground in the Falkland Islands will be regarded as being in support of the illegal occupation and accordingly is liable to attack."

The blockade is the latest in a series of rhetorical steps and military action undertaken by Britain since Argentine forces seized the Falklands on April 2. Thatcher dispatched a naval task force April 5 and declared the maritime exclusion zone a week later. Last Sunday, the vanguard of the task force engaged in its first action, retaking South Georgia Island, 800 miles east of the Falklands.

The military moves have stymied, at least for the time being, U.S. efforts to find a diplomatic solution to the crisis. Argentina postponed talks in Washington between U.S. Secretary of State Alexander M. Haig Jr. and Argentine Foreign Minister Nicanor Costa Mendez following the British action, although both countries say they are studying the new U.S. proposals.

The new blockade is designed to close the airport at Port Stanley, the main town on the sparsely inhabited islands. If effective, the blockade would cut off daily flights by Argentine military and civilian aircraft supplying as many as 10,000 troops on the islands.

The expectation of politicians and analysts here is that Britain will not attempt to starve out the Argentine garrison. Some suggested British forces would attack soon, possibly by this weekend.

One possibility is that Britain could use its aging, long-range Vulcan bombers to attack the airport. The crews of the Vulcans, due to be phased out as nuclear bombers, have been undergoing training in conventional warfare since the Falklands crisis began. A Defense Ministry spokesman declined to comment on possible use of the Vulcans but said the crews have been dropping 1,000-pound bombs on Garvie Island in Scotland in practice runs this month.

Although Argentina has air superiority over the task force, a Thatcher aide said Britain would not impose the blockade "if it could not enforce it."

The British task force, which includes two aircraft carriers, two assault ships and numerous destroyers and frigates, has 20 Harrier jump jets, 42 helicopters and a variety of missiles with a range of as much as 30 miles.

With concern about bloodshed increasing in Parliament, the Defense Ministry reported the death on Monday of an Argentine prisoner on South Georgia. A ministry statement said a board of inquiry had been established to investigate this "serious incident" and "will complete its deliberations as soon as possible." No details were released on the death.

The Thatcher government had emphasized Monday that nobody on either side was killed in the recapture of South Georgia, and said 156 Argentine military prisoners were being held under conditions of the Geneva Convention. Argentina returned without incident all British troops captured when it seized the Falklands.

The American peace proposals sent to Buenos Aires and London were described as quite similar to those Foreign Secretary Pym brought back from his recent talks with Haig in Washington. They still contain "difficulties" for the Thatcher government, according to British sources, who acknowledged, however, that they might look "less unattractive" if Argentina accepted them and began withdrawing its troops from the Falklands.

Thatcher is continuing to insist on a complete Argentine withdrawal before the British fleet would be pulled back, restoration at least temporarily of British administration on the islands, and a guarantee of the right of self-determination for the Falklands' 1,800 residents in negotiations over the islands' future.

She said this week she also would accept a U.S. or international peace-keeping force to supervise the Argentine withdrawal, along with the flying of Argentina's flag over a mission on the islands. These proposals are believed to be contained in the American plan, along with some form of sovereignty referendum for the Falkland Islanders.

Correspondent Diehl added the following from Buenos Aires:

Argentine military officials offered no immediate reaction to Britain's announcement of its intent to impose a new blockade. But London's statement appeared to ease tension in political circles here, where an imminent British landing on the islands had been expected.

High military officials and government spokesman maintained their silence over the military situation while the Argentine media continued to circulate reports both of expected British action in the Falklands and of continued Argentine resistance on South Georgia.

Reports here, citing unnamed military sources, have said that specially trained Argentine commando teams have continued to carry out attacks on British forces on the mountainous island seized Sunday.

The reports came amid continued denunciations by the junta and in government-controlled television of what is called a "psychological war" raging between Britain and Argentina.

The Argentine government still has not conceded the loss of South Georgia or British reports on light casualties. But Foreign Ministry officials did confirm that they had been notified by the Swiss Embassy here of the death of an Argentine soldier captured by the British. The Swiss have represented British interests in Buenos Aires since Argentina and Britain broke relations.

With the military junta here holding an unprecedented series of marathon sessions, Argentine analysts said the government faced a difficult choice between military and diplomatic options.

As the main body of the British fleet, including its two aircraft carriers, falls within the range of the more than 100 sohisticated jet fighters based on the Falklands and Argentine coastal bases, the government here must decide whether to launch an air strike on the fleet before it can carry out action against the islands.

Analysts here noted that a first strike might be in Argentina's best military interests, as it would give Argentine forces the initiative in what is seen as a relatively balanced match.

However, such a move would deprive Argentina of its current diplomatic advantage as a country facing a powerful attack, and would likely involve a high military cost even if successful. As a result, the government was expected to hold off on offensive military action while continuing to remain open to a negotiated solution.