The Reagan administration believes it knows the identities of El Salvadoran security officers who murdered four American women missionaries, but efforts to charge them have been hampere by resistence from Salvadoran military leaders and by concern that the available evidence is insufficient to win conviction.

The situation poses a potentially agonizing problem for the administration, which has staked much of its prestige on backing El Salvador's civilian-military junta with U.S. arms and military advisers in its struggle against what the administration contends are communist-supported guerrillas.

Opponents of the administration's policy have argued that the Salvadoran government also has been guilty of mass murder and oppression and that the United States has put itself in the position of supporting an unsavory regime.As a result, solving the murder of the missionaries has become an important test in U.S. public opinion about whether the administration can make good its promise to push the junta toward internal reform.

Administration sources said yesterday that leads developed by the FBI and turned over to Salvadoran officials have left little doubt, in a non-legal sense, about who killed the three Roman Catholic nuns and a lay worker Dec. 2. But, the sources added, the evidence gathered so far is not considered sufficient to pursue a successful prosecution in the Salvadoran courts.

That, the sources continued, has created problems for U.S. efforts to prod President Jose Napoleon Duarte's government into formally charging those involved. According to the sources, powerful elements within the Salvadoran military and security forces have seized on the shaky nature of the evidence to resist bringing the suspected killers, some of whom are members of the security forces, to trial.

As a result, the sources said, the administration is quietly but firmly pressing the Duarte government to keep its investigation going in the hopes that it can come up with more evidence on which to build a more solid case. At the same time, the sources added, the United States is maneuvering, through behind-the-scenes threats and persuasion, to put the Salvadoran military on notice that the suspects must be tried if the requisite evidence is obtained.

The matter is considered so delicate that the State Department retreated into a stance of uncomfortable near-silence yesterday after Robert White, the former U.S. ambassador in El Salvador, charged Thursday that the department has known for weeks that six members of the National Guard have been arrested in connection with the killings.

White, a career officer who was fired and forced into retirement by Secretary of State Alexandria M. Haig Jr., said at a Rochester, N.Y., press conference that the administration and the Salvadoran government are guilty of a coverup in the matter.

Asked yesterday about White's charges, department spokesman Dean Fischer said, "We understand that a number of persons, including members of the security forces, are under investigation." He refused further comment.

However, other administration sources confirmed that the suspects are six members of a Salvadoran National Guard patrol who were known to have stopped the missionaries' vehicle as they drove down the San Salvadoran airport toward the city. The sources said it was only in recent days that enough evidence was developed to harden suspicions about the patrol's complicity, and they added it still is not clear whether all six members took part in the killing.

The sources conceded that the United States now must convince the Salvadoran military, which has been accustomed through decades of dictatorial rule to killing people with impunity, that it risks losing U.S. support if the killers are not brought to justice.

Greatly complicating that task, the sources added, is the insubstantial nature of the evidence, which they said is based largely on ballistics and fingerprints obtained from the missionaries' vehicle and processed to be specific, the sources said the totality of the evidence collected so far would be insufficient even to obtain indictments in U.S. courts and is regarded as falling short of the requirements for a conviction under Salvadoran law.

However, they added, despite efforts to project a low profile about direct U.S. involvement, the FBI still is working with Salvadoran authorities on developing more evidence of an unspecified nature in hopes of bolstering a prosecution case. The sources cautioned that even if this evidence is obtained, the case will be what lawyers consider a longshot in terms of winning convictions, but they added it should at least be sufficient to demand that the Salvadoran government formally charge the suspects.