Reagan administration officials played a role in helping U.S.-backed Nicaraguan insurgents to buy an anti-aircraft missile that downed a Nicaraguan helicopter in December, according to administration officials and congressional sources familiar with the case.

Senior officials, including Secretary of State George Shultz, have denied charges by the Sandinista government in Managua that the United States provided the rebels with the heat-seeking, surface-to-air SAM-7 missile that downed the Soviet-supplied Mi8 Sandinista helicopter on Dec. 2. Twelve Nicaraguan soldiers and two Cuban pilots were reported killed.

Knowledgeable sources, while confirming that the United States did not supply the Soviet-made SAM-7 missiles, said officials who monitor the rebels' activities passed information to the rebels through third parties on how to secure the weapons through foreign arms dealers.

In another instance of U.S. involvement, administration sources said that while the contras awaited delivery of the missiles, U.S. officials in Central America, acting on their own, described for the rebels the vulnerabilities of the Russian helicopters so they could use their new weapons effectively.

The rebels, or contras, reportedly purchased their first missiles from European arms dealers in February or March, took delivery in April at their Honduran camps and were trained in their use by private American arms experts.

"It wasn't a question of U.S. officials meeting secretly with guerrillas in the basement of the White House in the middle of the night and telling them 'Now boys, you gotta get surface- to-air missiles' and 'don't worry, we'll get them for you, ' " said one official aware of contra activities.

'INDIRECT HINTS'

"But there were enough indirect hints from officials through third parties as to where they could go and get the weapons and training. The whole thing would have been impossible without some sort of U.S. participation."

The part American officials played was so discreet that it is unlikely that it could be conclusively established that they circumvented a congressional ban on assisting the contras, which was in place when the missiles were acquired in early 1985, the sources said.

"Those who helped covered their tracks so well that enough deniability was preserved, " said a congressional source who was briefed by U.S. intelligence officials. He said there is no evidence the administration had a policy of supplying missiles to the rebels.

HOW CONTRAS GOT SAMS

To understand the context in which U.S. officials became involved, it is necessary to review chronologically how the contras came to acquire the SAM-7s.

The process began election night, Nov. 6, 1984, when American intelligence sources reported that a Soviet merchant ship, the Bakuriani, was en route to Nicaragua reportedly carrying crated MiG-21 combat jets.

The report was wrong, but the crates did contain a new weapon for the Sandinistas: the first of several sophisticated Mi24 helicopter gunships that the Soviets have used against rebels in Afghanistan.

Within days U.S. officials warned the contras that the Sandinistas planned to employ the Mi24s against the Nicaraguan guerrillas and suggested that they needed anti-aircraft weapons.

At the time, contra leaders Adolfo Calero and Enrique Bermudez raised the possibility of obtaining U.S.-made Redeye surface-to-air missiles but the effort failed because of the congressional ban against supplying the contras with weapons, the sources said.