U.S. efforts to promote a gradual liberalization by Chile's military rulers are proving ineffective and have increased concern among South America's new democratic governments, diplomats and politicians here say.

During the past two years, the Reagan administration has sought to pressure both the government of Gen. Augusto Pinochet and its democratic opposition to negotiate a consensus move toward democracy, using a military-backed constitution as a starting point.

Progress under this formula, however, now appears even more remote than when mass protests against Pinochet's rule began in May 1983. They were followed initially by a loosening of the absolute military control, including permission for hundreds of opposition politicians to return from exile. But Pinochet then reversed the trend and, citing an outbreak of political violence nine months ago, imposed a state of siege.

Since then, democratic opposition political parties, largely muzzled, have rejected U.S. proposals for accepting military guidelines as unrealistic and antidemocratic.

U.S. officials have responded to the impasse by stepping up both public and private pressure on Pinochet to lift the state of siege and respect his own commitments to change. At the same time, the United States consistently has disdained the opposition's alternative plans for democracy and rejected suggestions that Pinochet has become an insuperable obstacle to transition.

This position has led to increasing tension in U.S. relations with both Pinochet and the parties and helped to make Washington's policy in Chile important to other Latin American governments concerned about the implications for survival of democracy generally.

"There has been a change of U.S. tactics with Pinochet, but not a change of strategy," said Sergio Bitar, a politician who spent a decade in exile after the military coup of 1973. "The United States government continues to believe that the most reasonable solution lies in a transition with Pinochet until 1989. What they don't understand is that this is simply not viable."

In recent months, Argentine President Raul Alfonsin, Venezuelan President Jaime Lusinchi and the late president-elect of Brazil, Tancredo Neves, separately raised concerns about Chile in personal meetings with Reagan, officials here said. Each urged a tougher approach toward Pinochet and more support for his democratic opposition, according to local political sources.

While U.S. officials argue that their proposals are motivated by pragmatism, many democratic leaders both in and out of Chile argue that the Reagan administration's approach has been twisted by its interest in political gains.

"The United States is afraid of an open democratic system in Chile," said Ricardo Lagos, a leader of the Socialist Party. "They think that their kind of formula will result in the left being excluded from the system. That, and not democracy, is their most important objective here."

Pointing to increasing support for violence by sectors of the illegal but large Communist Party, U.S. officials frequently have expressed concerns that antigovernment mobilization, even if nominally led by moderate groups, could have the effect of facilitating a return to power by the left. A Marxist coalition ruled under democracy here from 1970 until the 1973 coup.

Non-Marxist leaders, however, argue that these U.S. concerns are greatly exaggerated and ignore the traditional role of leftist parties in Chilean democracy.

"The thinking of the State Department leaps very quickly from antigovernment mobilization to political instability to Communist dictatorship and the Soviet Union," said Bitar. "And the perceived risk is more important to them than the democratic process." Bitar was a minister in one of the Cabinets of the late president Salvador Allende. About 2,000 exiles still are refused permission to return.

Formally, U.S. officials maintain that the Reagan administration does not favor a particular political formula in Chile. Rather, as Deputy Assistant Secretary of State James Michel recently told a congressional subcommittee, "the primary objective of the administration is to . . . encourage protransition forces in the government and pronegotiation forces in the opposition to reach a consensus on a democratic transition timetable."

In practice, however, top U.S. officials consistently have urged Chilean opposition leaders to accept the military's 1980 constitution as a basis for any negotiations. That charter, sanctioned in a plebiscite condemned as unfair by the opposition, spells out a gradual move toward democracy until 1989. At that time, another plebiscite would determine whether the country should return to free elections or whether Pinochet or another military-appointed president should rule for eight more years.

After 1990, the political system established by the constitution would continue to ban Marxist parties, grant exceptional powers to the president and provide the military with permanent privileges ranging from the direct appointment of several senators and key judges to an institutionalized political oversight role through a "national security council."

Opposition parties have labeled both these future guidelines of the state and the planned transition as undemocratic and "illegitimate" and have made rejection of the constitution a fundamental part of their own platforms. Pinochet, in turn, has declared that he will not tolerate any political group that does not accept the constitution.

U.S. officials have argued that the Chilean opposition groups, including the centrist Democratic Alliance, should accept the constitution as a starting point and then work within the military's system, seeking changes in both the permanent charter provisions and the form of the transition.

In a meeting with Chilean politicians earlier this month in Washington, Undersecretary of State Michael Armacost stressed this argument by comparing the Chilean situation to the Philippines. According to several persons present at the meeting, Armacost's point was that just as political opponents of Philippine President Ferdinand Marcos had gained ground by agreeing to participate in parliamentary elections, Chilean activists could progress by negotiating on the basis of Pinochet's constitution.

In other meetings with democratic leaders, U.S. officials also have echoed government calls on the parties of the Democratic Alliance to sever all ties with the Marxist Democratic Popular Movement, excluding it from any role in the transition process, according to opposition activists.

This U.S. strategy has embittered a number of opposition politicians, who say they are convinced that a transition to full democracy is not possible under the present constitution, especially if leftist groups are excluded. These groups have sought since 1983 to mobilize pressure on the government to organize elections before 1989 while allowing a democratically chosen constituent assembly to decide on the constitution.

"The Americans always say that what we are asking is unrealistic," said Manuel Antonio Garreton, a Socialist activist. "But what they are asking is that we try to change the constitution completely from the inside without saying that's what we are doing. They are asking that we legalize the dictatorship. And that is what is really unrealistic."

The opposition leaders say that U.S. preoccupation with the left overlooks the more fundamental problem of Pinochet's behavior. Even if they were to accept the U.S. formula, they say, the government has made negotiation impossible by abruptly ending liberalization policies and by banning the opposition press and party meetings under the state of siege.

Pinochet, who seemed willing to consider a negotiated transition during the last months of 1983, has declared since then that he will not negotiate changes in the constitution with his opposition.

U.S. officials say they agree that the president has shown few signs that he is willing to carry out the constitution as it currently stands, much less accept revisions. But they argue that the Reagan administration has few alternatives other than to continue pressuring both the government and the opposition to accept each other's legitimacy.

"There are certain facts that have to be accepted," said one source familiar with U.S. views. "And one is that there is a constitution that was approved and is considered legitimate by the armed forces. You have to work with that if you are ever going to encourage a consensus."