The United States is supplying weapons to rebel forces battling Soviet troops in Afghanistan, according to reliable sources.

The weapons, presumably being slipped across Pakistan's rugged 1,400-mile frontier with Afghanistan, are said to be mostly small arms and relatively simple anti-tank weapons that give a solider the capability to knock out an armored vehicle.

The smuggled arms are said to be Soviet-built, which helps disguise the source of supply.

The covert U.S. weapons supply, which sources say began after the Soviets invaded Afghanistan of Dec. 24, is said to be "neither big nor dramatic."

Yet it does reflect a Carter administration decision to try to help the beleagured and vastly outgunned rebel forces while raising the cost to the Soviets of their action.

Though it is not known precisely how the supply is being carried out, the operator presumably is the Central Intelligence Agency. Key committees of Congress responsible for overseeing covert activities have been kept informed on the administration actions by the State Department and CIA.

There are many areas outside the Soviet Union where Soviet-build weapons are in use and can be acquired. This includes a sizable underground arms market flowing west from Eastern Europe; Africa, where Cuban forces use and lose such weapons, and countries such as Egypt and China that were heavily supplied with Soviet arms but have since cut their military ties to Moscow.

The decision to supply arms, even in limited quantities, is a significant step beyond the aid the United States was providing Afghan insurgents prior to the Soviet invasion.

Moscow's quest for a suitable pro-Soviet government to help install in Kabul in the year preceding the invasion involved a sizable military "advisory" presence. That, in turn, sparked increasing anti-Soviet resistance.

U.S. covert aid prior to the December invasion, according to sources, was limited to funneling small amounts of medical supplies and communications equipment to scattered rebel tribes, plus what is described as "technical advice" to the rebels about where they could acquire arms on their own.

The U.S. press also has reported that China is supplying arms aid to the rebels.

The Egyptian Defense Ministry in Cairo yesterday announced that it was training Afghan rebels in guerrilla warfare and plans to arm them and send them back to Afghanistan to battle the Soviet forces.

Moscow has consistently cited what it calls the "machinations of the aggressive imperialist forces" of the United States, China and Pakistan to rationalize its invasion. Washington rejects this, saying that there was no external threat to Afghanistan and that the real threat to that country comes from the Soviet army of occupation.

Thus the covert operation, even on a small scale, has always been sensitive.

The issue is especially sensitive now because of concern that the Soviets could use it to justify attacks on border regions with Pakistan. Several refugee camps in Pakistan are filled with thousands of Afghans who have fled their homeland, and some of them return home to fight.

On the other hand, the covert transactions could ultimately raise questions about whether the secret aid to the rebels, while helping to harass and tie up the Russian forces, may also hinder the departure, which is the administration's stated objective.

In recent years, Congress has tended to prohibit covert arms aid to beleaguered countries. The Afghanistan action, however, takes place in a new context -- global tensions unleashed by the Soviet invasion.

U.S. officials have previously said privately that the insurgents were actually well supplied with arms by units of the Afghan army that defected to the rebel side. The U.S. need to slip some additional arms to the rebels would seem to contradict that view, unless the arms available inside Afghanistan are not suitable for use against the Soviet armored units.

Though the State Department has consistently declined to commet on questions about what, if any thing, the United States is and has been doing in Afghanistan, there have been a number of public hints of some U.S. action.

The clearest indication of some kind of pre-invasion acitivity inside Afghanistan came to light on Jan. 9 -- two weeks after the invasion -- during an appearance on NBC-TV's "Today" show by Sen. Birch Bayh (D-Ind).

The NBC questioner referred to the Soviets having set up a regime friendly to Moscow a year or more ago, and pointed out that there are Afghans opposed to that regime. "Can you say whether we have in any way been trying to help them?" he asked.

"I think I can say," the senator replied, "that when a significant number of people in Afghanistan were determined to try to exert themselves and to try to have some say in what kind of government Afghanistan should have, and not have it imposed upon them by the Soviets, we did take certain steps to help them do what any group of citizens should be able to do in a country."

Asked recently about what he was referring to, Bayh, through his spokesman, declined to elaborate.

Bayh is chairman of the Senate Select Committee of Intelligence, which has been kept informed of U.S. actions, sources say.

Early in January, and administration official, who asked not be be identified, told reporters that the United States was considering ways to assist the Afghan rebels and said, "We are not ruling anything out."

On Jan. 5, The Boston Globe reported a "hash-hush decision that the United States and perhaps China, will do everything possible to slip weapons to the Moslem insurgents in Arfghanistan. . . ."