The Reagan administration yesterday sharply stepped up pressure on wavering congressional committees to let it increase military aid to El Salvador, and without imposing binding conditions. The deadline for Congress to act to prevent the increase is Thursday, and the administration seemed to be gaining ground last night.

In testimony before a Senate Appropriations subcommittee, Secretary of State George P. Shultz steadfastly adhered to previous administration statements that the United States would support negotiations between the Salvadoran government and the guerrillas over upcoming elections, but he provided no details on how to assure the security of the leftists or how to guarantee a fair election.

Key members of Congress have demanded that the administration produce specific proposals to bring the guerrillas into negotiations.

However, Sen. Daniel K. Inouye (D-Hawaii), who predicted earlier that the subcommittee would cut the administration's $60 million request to $23.7 million, yesterday said he did not appear to have the votes for his proposal.

Chairman Robert W. Kasten Jr. (R-Wis.) said the subcommittee might trim the administration's request by $5 million or $10 million and add "some language" encouraging judicial reform and limiting the number of U.S. trainers in El Salvador to 55.

In an effort to avoid a divisive public vote, Kasten said he would poll subcommittee members individually to obtain a "consensus" position before Thursday, when the request to reprogram $60 million in funds for other countries will take effect unless vetoed by the Senate or House Appropriations foreign operations subcommittees or the Senate Foreign Relations Committee.

Although the House subcommittee may cut the amount Reagan has requested by as much as half, several members said, the administration will have won a significant victory if it gets the money without strong conditions attached.

Chairman Clarence D. Long (D-Md.) earlier had called on the administration to go beyond "verbal assurances" to guarantee the safety and participation of exiled Christian and Social Democratic leaders, now operating as the guerrillas' political arm.

Yesterday, Assistant Secretary of State Thomas O. Enders provided Long's subcommittee with "confidential" papers on the political and military situation in El Salvador, but subcommittee members said there were no new proposals.

Challenging the proposal by Sens. Inouye, Nancy Landon Kassebaum (R-Kan.) and Christopher J. Dodd (D-Conn.) that military aid be capped at $50 million a year until the Salvadorans agree to "unconditional" negotiations with the opposition, Shultz said: "We will not support negotiations that short-circuit the democratic process and carve up power behind the people's back."

He added that discussion of a peace initiative "now centers on the possibility of a meeting of foreign ministers of the five Central American countries"--Costa Rica, El Salvador, Honduras, Nicaragua and Guatemala--"observed by the foreign ministers of five other countries in the region, Colombia, the Dominican Republic, Mexico, Panama and Venezuela."

At the moment, he said, "the vast bulk of arms are coming through Cuba and Nicaragua to the Salvadoran guerrillas. Some are American arms captured in Vietnam. They have been identified by serial number."

Sens. Arlen Specter (R-Pa.), Alfonse M. D'Amato (R-N.Y.) and J. Bennett Johnston (D-La.), all past supporters of the administration, criticized the Salvadoran government for failing to prosecute the men indicted in the murder of four American churchwomen and for allowing uncontrolled killing of civilians despite objections from the United States.

"They're thumbing their nose at us," Johnston said, suggesting that no aid be given unless the Salvadorans agree to try the accused by a specified date.

Shultz said he does not want conditions attached to the aid, but said of the Salvadorans: "In the end, if they don't clean up their act, the support is going to dry up. They know that. They've been told that. It's going to happen."