A military officer assigned to the U.S. Embassy in El Salvador served as a conduit for a group of American civilians to provide military-related supplies, including ammunition clips and camouflage uniforms, to the Salvadoran army, U.S. officials acknowledged yesterday.

Army Maj. C.A. McAnarney, a logistics officer at the embassy, confirmed yesterday that he forwarded nearly a dozen shipments to the Salvadoran government after receiving them by mail from Tom Posey, a former marine in Alabama who heads a civilian group dedicated to fighting communism in Central America.

Last November, McAnarney also translated into Spanish Posey's initial offer to provide the supplies to El Salvador at no charge.

The State Department yesterday asked the Customs Service to investigate whether Posey violated federal law by sending certain military-related supplies to El Salvador. The request came as several federal agencies played hot potato with a growing list of questions about the extent to which the Reagan administration may have aided Posey's efforts to help El Salvador's army and anti-government rebels in Nicaragua.

Administration officials took no steps to block Posey's shipments of military-related supplies to El Salvador, although the State Department said it had issued him no export license.

Such licenses are required for export of items on the department's munitions control list. An official said the list includes empty ammunition clips, ammunition pouches and camouflage uniforms, which were among the items Posey sent to El Salvador.

In addition, the Treasury Department did not notify the State Department that Posey had declared, on a Treasury application to become a dealer of firearms in the United States, that "I plan to buy weapons and ammo to send to El Salvador."

Treasury officials said that they granted Posey the dealer's license because his plan to send weapons to a foreign government was the State Department's responsibility, not theirs.

Since two American members of Posey's group, Civilian-Military Assistance, were shot down and killed in a rebel helicopter over Nicaragua last weekend, the administration has gone out of its way to insist that it provided no assistance to the group. But there is also evidence that the administration facilitated the group's efforts.

Sen. Jim Sasser (D-Tenn.) said yesterday that the administration "clearly had no interest in hindering Posey in his efforts to ship weapons and military supplies to El Salvador. The Treasury was put on notice by his application for a license. Two or three agencies of the government knew about it and took no effort to restrain him."

Sasser said that Posey "was encouraged by their acquiescence in what he was doing . . . . It raises a question, clearly, about whether the laws were being properly enforced."

"If some group wanted to ship arms to Cuba, would the State Dertment respond in the same way?" a House Democratic aide asked. "Of course the administration would find a way to stop it."

According to letters obtained by Washington Post special correspondent Brian Barger, McAnarney wrote Posey last Nov. 3: "Attached you will find a letter in which you and your organization make a formal offer to the Salvadoran Government to provide them with the listed equipment at no cost to them. Please sign and return original plus one copy in the enclosed envelope. We will forward it to the Salvadoran Government."

From November through March, according to the correspondence, Posey sent McAnarney at least 11 shipments of supplies for the Salvadorans. The letters said that the shipments included pistol belts, rifle-cleaning kits, ammunition clips and pouches, ponchos, first aid kits, canteens, field glasses, protective vests, camouflage uniforms and jungle boots.

"Here is another sample of the equipment to be sent down their sic ," Posey said in a typical letter. "This equipment is at no cost to the El Salvador government . . . . Please give it to the proper people."

U.S. Embassy officials in El Salvador initially told Washington Post correspondent Edward Cody that they did not know what was in the boxes sent by Posey. But McAnarney, through a spokesman, later confirmed that he had received the shipments from Posey and forwarded them to the Salvadoran army.

Embassy spokesman Donald Hamilton described Posey as a gadfly who received minimal help from the embassy. "I think all of us who dealt with him felt he was a pest," Hamilton said.

Posey told McAnarney March 11 that he would stop mailing him the packages, which were insured and sent through an Army post office in Miami. "I have made contact with an El Salvadoran Air Force officer who has given me the information as how to get equipment to his country other than mail," he wrote.

A U.S. military cargo plane was used this summer to haul medical supplies donated by private groups to El Salvador and Honduras.

Posey also notified McAnarney that his group offered similar assistance to the Nicaraguan Democratic Force (FDN) in Honduras. "They gave us in writing a letter saying they would take our help," Posey wrote.

McAnarney apparently was the second embassy official in El Salvador to help Posey, who said that another, unidentified, official introduced him to a top Salvadoran colonel.

Posey said yesterday that no official ever told him he needed a State Department export license for such items as ammunition clips.

State Department spokesman John Hughes reiterated yesterday that "there was no U.S. government collaboration with the activities of this group. As they have consistently stated, they were acting on their own."

But, Hughes added, "We're looking into the suggestion that there may have been individual American officials who may have had some contact with these people."

There was considerable buck-passing about the Posey case yesterday. Hughes said the State Department has asked the Customs Service "to initiate an investigation into whether there is a willful violation of . . . the Arms Export Control Act in this instance."

A Postal Service spokesman said Customs is responsible for monitoring weapons sent through the mail. But Customs spokesman Dennis Murphy said his agency has been unable to inspect military items sent abroad because the Postal Service has refused to allow its agents to inspect foreign-bound mail.

Lonnie Muncy, speaking for Treasury's Bureau of Alcohol, Tobacco and Firearms, said it is not bureau policy to tell the State Department when an applicant declares that he will send weapons abroad.

Muncy said the bureau had no choice but to issue Posey a gun dealer's permit, which can be used only domestically. He said permission can be denied only if an applicant has a criminal record or has been committed to an institution, and that no background probe is conducted.

"It would be a routine application," Muncy said. "If he's not prohibited from having a gun, he'd be entitled to the license. The bureau would have no option."

Muncy said an applicant is supposed to have a legitimate manufacturing or distribution business, but that as far as BATF is concerned, it is "immaterial" if that business is selling or giving weapons to foreign countries.

Quirks in the law also help explain why Customs allowed Posey and several colleagues, on a number of trips to Honduras, to take the legal limit of three guns and 1,000 rounds of ammunition apiece. Although the Posey group used the weapons to train Nicaraguan rebels, Customs officials say they assume that travelers are using the weapons for personal protection or sporting purposes.

"We don't ask," Customs spokesman Christine Frazer said. "We don't conduct any inquiry as to what they're going to use it for."

Another federal agency is involved in the case. According to sources, the FBI is examining whether Posey violated the Neutrality Act, which bars private citizens from joining efforts to overthrow a foreign government with which the United States is not at war.

In a letter to the chairmen of the House and Senate judiciary committees, the Council on Hemispheric Affairs contended that an independent counsel ought to be appointed to investigate the activities of Civilian-Military Assistance.

Laurence Birns, director of the liberal research group, contended that the Reagan Justice Department is too "politicized . . . to independently assess the legality of Reagan administration actions towards Nicaragua."