The Reagan administration, after hesitating for years to send sophisticated U.S. weapons to insurgent forces in the Third World, has begun supplying several hundred Stinger missiles covertly to anticommunist rebels in Angola and Afghanistan, informed sources said yesterday.

The decision, which has been closely held among the president's national security affairs advisers since it was made earlier this month, marks a major shift in U.S. policy. Shipments of top-of-the-line American arms to such insurgents had been barred in favor of furnishing largely Soviet- and Chinese-made weapons bought on the international arms market or from U.S. allies.

The change in policy is certain to broaden involvement of the Central Intelligence Agency in Third World conflicts and appears likely to escalate the fighting in Afghanistan and Angola, where Soviet helicopter gunships have inflicted heavy casualties on rebels forces in the past year.

The shift occurred after activists in the Pentagon and the CIA, backed by conservatives in the Senate and elsewhere, overcame opposition by officials in the State Department, as well as some in the CIA. Opponents of the change long have argued that introduction of U.S.-made weapons into Third World conflicts escalates those struggles into U.S.-Soviet confrontations. Those situations pose touchy problems for neighboring states attempting to maintain a neutral diplomatic posture while providing a route for U.S.-backed arms shipments.

Introduction of such weapons also makes it more difficult for the U.S. government to maintain a posture of "plausible deniability" of its involvement in such conflicts.

A White House spokesman said the administration had no comment on whether Stinger heat-seeking antiaircraft missiles have been provided to rebels in the two countries. Nor would he comment on reports that Stingers might also be sent to the U.S.-backed contras, or counterrevolutionaries, fighting the Sandinista government in Nicaragua.

The Stinger decision followed the Feb. 25 recommendation of an interagency committee made up of senior representatives from the State Department, CIA, Defense Department and the National Security Council staff. The committee, which meets periodically in the White House situation room or in Room 208 of the Old Executive Office Building, is charged with planning and coordinating all CIA covert paramilitary operations.

Over the past year, the interagency review of U.S. covert paramilitary operations concluded that Soviet-backed forces were employing more lethal weaponry and more aggressive tactics against mujahadeen rebels in Afghanistan and against the guerrilla army of Jonas Savimbi in Angola.

One intelligence estimate indicates that roughly one-third of Soviet special forces units, trained for counterinsurgency and night combat roles, have been deployed to Afghanistan where they have inflicted heavy casualties.

In Angola, a large column of Soviet-made tanks and armored vehicles backed by helicopter gunships and MiG21 and MiG23 jet fighters are poised for an offensive expected in the next 60 days to rid the Marxist central government of Savimbi's 10-year-old insurgency.

Rebels in both countries have been opposing the increased air threat with Soviet-made, shoulder-fired SA7 missiles and have complained that their range -- less than two miles -- is not sufficient to thwart "stand-off" attacks by heavily armored Soviet gunships. The rebels also have complained about the reliability of Soviet SA7s, whose battery-driven electronics apparently are subject to frequent failure.

The Stinger, a state-of-the-art antiaircraft missile made by General Dynamics Corp. and supplied to only a few U.S. allies, is a far more lethal weapon than the SA7 the United States has been supplying to the Afghan rebels. The Stinger has a range of up to five miles and employs a supercooled sensor to lock on to aircraft heat emissions and is not easily fooled by decoy flares fired by Soviet helicopters.

In a letter to Reagan last month, a group of conservative senators estimated that Stingers could improve the "kill" capabilities of rebel forces facing Soviet military aircraft by three to ten times.

The CIA's clandestine service chief, Clair George, was described by sources as a strong proponent of the Stinger decision. George, who has been credited by CIA Director William J. Casey with rebuilding the agency's paramilitary arm, represented the CIA in the interagency deliberations. Casey visited Africa this month to meet with Savimbi and assure him that "effective" antiaircraft weapons were on the way, sources said.

The administration has been under pressure for months from conservative senators and political action groups to provide U.S. weapons to the anticommunist insurgents. The CIA and State Department have been criticized by these groups for dragging their feet.

A key event in the Stinger decision, according to sources, was a meeting on March 5 between Secretary of State George P. Shultz and a group of mostly conservative senators led by Senate Majority Leader Robert J. Dole (R-Kan.). During the 70-minute meeting in Dole's office, Shultz was pressed four different times to move quickly to provide Stingers to Angolan rebel leader Savimbi and to the Afghan mujahadeen resistance. Each time, according to sources, Shultz asked the senators: "Are you sure you want me to go back to Bill Casey and tell him you want Stingers?"

All nodded and said, "Yes," the sources said.

In the week following this meeting, top-secret presidential authority was given to ship several hundred Stingers to Angola and Afghanistan, according to sources. These shipments arrived during the past week, a source said.

Some conservatives have voiced private criticism of the Stinger decision, saying that the original proposal to help "freedom fighters" with American weapons called for thousands of U.S. antiair and antitank missiles to be provided to U.S.-backed insurgents in Nicaragua and Cambodia as well as Afghanistan and Angola.

An administration official said the House and Senate intelligence oversight committees were notified of the Stinger decision late last week.