The Reagan administration, seeking to protect its policy in El Salvador from a new round of criticism because of the latest incident of violence there, has blamed one lelement of that country's infamous "security forces" for the killing of 30 civilians last week.

A State Department official, speaking to reporters on the condition that he not be identified by name, said the United States now understands the officials of El Salvador's treasury police have confirmed that some of their forces were involved in the shooting.

"The United States is providing security assistance only to the regular Salvadoran armed forces, which have a record of conducting themselves more professionally in dealing with the people," the official noted.

In the aftermath of the shootings in the San Salvador suburb of Soyopango, American officials have sought to distinquish between "regular" army units in El Salvador and the country's security forces, which consist of the national guard, the national police and the treasury police.

Beyond the question, the latest shooting incident has complicated the administration's task of maintaining congressional support for its policy of providing military as well as economic aid for the civilian-military government of El Salvador that is headed by Jose Napoleon Durate.

Even before the latest incident, there was strong cirticism of the Salvadoran government because of the killings last December of four American Roman Catholic women who were working as volunteers in El Salvador.

Elements of the treasury police are also thought to be involved in the killing of the four women, murders which have remained unsolved amid charges of a "cover-up" by the Salvadoran government.

The problem caused for the administration by the continued violence involving government forces was illustrated last week by a brief speech on the House floor by Rep. Jerry Lewis (R-Calif.), whose district includes Palm Springs, which is far from a bastion of anti-administration liberalism.

Lewis noted that he was part of a one-vote majority in a House Appropriations subcommittee that voted to uphold sending military aid to El Salvador this year. But, citing accounts of the killings of the civilians last week, Lewis added:

"These continued reports would indicate that we still have very serious problems of terrorism from both the left and the right in that country. I for one am calling upon the administration to review these actions and to give the Congress an analysis of exactly what took place in El Salvador in this instance . . . .

"As I review such (military aid) votes in the future, I will look for evidence that such assistance does not end up in the hands of terrorists on the right and become of benefit to those who seek a military dictatorship and the destruction of the dream for democracy in El Salvador."

Lewis' message seemed a clear warning to the administration that it cannot indefinitely count on the support of Republicans unless there is evidence that violence against civilians by Salvadoran government forces is being brought under control.

Privately, the administration is believed to be pressing the Salvadoran government to bring its security forces under tighter control, and warning of a possible erosion of political support in the United States for the aid program if the violence continues.

Publicly, it has sought in recent days to draw a more definite distinction between units like treasury police, which is widely feared in El Salvador, and the country's armed forces.

However, one complicating factor in drawing this distinction is that Col. Jose Guillermo Garcia is the country's minister of defense and public security, a combined post that puts him in charge not only of the army but also of the country's security apparatus, including the treasury police.

The State Department official who volunteered comments on El Salvador to reporters Friday said the treasury police "have been able to operate somewhat independently" in the country.

He said the U.S. aid going to El Salvador is intended to "support the regular armed forces," and that none of this assistance finds its way into the hands of the treasury police.