President Reagan's decision to send the Stinger antiaircraft missile to rebels in Afghanistan and Angola as a symbol of his commitment to their anticommunist cause is becoming increasingly controversial within Congress and the administration because of questions about the missile's complexity and concern about it falling into terrorist hands.

In early March, the president, largely at the insistence of conservatives in Congress and his administration, secretly approved the supply of a limited number of Stingers -- 250 to 350, according to sources -- to aid "freedom fighters" in Angola and Afghanistan.

Afghan rebel leaders visiting here this week said they had yet to receive Stingers on the battlefield and pleaded with administration officials for effective antiaircraft arms.

Sources close to the Angolan rebels say, however, that the weapon has already gone into service and shot down two Soviet-built Mi24 Hind helicopters and one MiG at the start of the latest government offensive to crush them. There has been no independent confirmation of this claim.

The decision to provide Stingers has reinvigorated the debate over the wisdom of distributing such a sophisticated hand-carried and shoulder-fired weapon to guerrilla groups over which the U.S. government has virtually no control. The administration recently had to eliminate Stingers from its $354 million arms sales package to Saudi Arabia because of congressional concern over the possibility they might find their way into the hands of Arab terrorists.

Sen. Dennis DeConcini (D-Ariz.) introduced an amendment to the Defense Department reorganization plan approved by the Senate May 7 that would have required strict monitoring of Stingers provided to the Afghan and Angolan rebels and safety provisions similar to those required of U.S. allies. His bill was defeated 62 to 34. An aide said DeConcini plans to submit another amendment requiring somewhat less rigorous controls when the defense authorization bill comes to the floor.

Pentagon officials, without taking direct issue with the administration's decision, have indicated some dissatisfaction, pointing out that the Army has only a limited supply of Stingers for its use. They also cited the complexities involved in the use and maintenance of Stingers.

Army Chief of Staff Gen. John A. Wickham said in an interview with The Washington Post yesterday that "we anguish over decisions" of which allies should get the weapon because of U.S. concern it will fall into the wrong hands. He also said that secrecy about the Stinger's basic technology has been compromised "overseas" and that the United States is having to upgrade the weapon continually to avoid new Soviet-designed countermeasures.

There are now three generations of the weapon -- the original Stinger, the Stinger-POST and, currently under development, the Stinger-RMP.

All three represent an enormous advance over the older Redeye missile because they have the ability to down a plane head on, rather than depending on engine heat for a target guide.

The Stinger-POST, for Passive Optical Seeker Technique, was developed to overcome flares that could be dropped by enemy aircraft to deflect the missile.

The Stinger-RMP, for Reprogrammable Microprocessor, was developed to allow the Army to change the missile's software guidance system easily by allowing it to be affixed outside rather than inside.

It is the first-generation Stinger that the administration is providing to the Afghan and Angolan rebels, according to published reports. Angolan rebel sources say they have had no trouble using any of the American arms provided to them. Sources close to the Afghan rebels, however, report that no U.S. training program was established for them and that the Pentagon and Central Intelligence Agency are at odds over who should do it.

Training is a key factor in the Stinger's usefulness because of its sophistication.

Richard L. Armitage, the assistant secretary of defense for international security, has underlined the difficulty American soldiers face in learning to use the Stinger. He told the Senate Foreign Relations Committee in April while testifying on the Saudi arms package that it requires 136 hours for an American soldier to gain proficiency with the missile.

An Army spokesman said roughly half of the 136 hours went toward training the user on how to identify friendly aircraft from enemy planes, a problem the rebel fighters generally don't face.

One other time-consuming effort was learning to sight a fast-flying plane and judge its distance. This exercise is done here on a Moving Target Simulator (MTS) inside a domed auditorium. The Stinger's effective range is roughly three miles, the spokesman said.

The Saudis, who possess 400 Stingers, said their tests have shown that the missile does not have the performance rate claimed by the U.S. military and that it requires so much training its value is limited.

Saudi officials concluded from tests of similar weapons that an upgraded version of the Soviet SA7 made by the Chinese was the easiest to use and most effective. Wickham said that the Stinger requires "substantial training and refresher training" and "like any weapons system, it does have some limitations."