Secretary of State George P. Shultz is scheduled to visit a Cambodian insurgent camp on the Thai-Cambodian border Tuesday, a sign of growing U.S. support for noncommunist rebels fighting the communist regime installed in Cambodia by Vietnam. But according to informed sources, Shultz's public gesture is actually a complement to a program of covert CIA aid to the same insurgents.

According to these sources, the Central Intelligence Agency has been covertly providing millions of dollars a year since 1982 for nonmilitary purposes to two noncommunist Cambodian resistance groups, including more than $5 million this year.

The CIA's aid is funneled through Thailand, the sources said. The Reagan administration's goal is to strengthen the two noncommunist resistance groups' position in their loose coalition with the communist Khmer Rouge.

The Khmer Rouge, under Pol Pot, were responsible for killing as many as 3 million Cambodians while they ruled the country from 1975 to 1979. Vietnam invaded Cambodia, removed Pol Pot and installed a puppet regime in Phnom Penh in 1979.

There is a congressional ban on aiding the Khmer Rouge, but liberal Democrats in the House have encouraged an effort to give aid openly to the noncommunist insurgents, proposing a grant of $5 million in military assistance this year. Several intelligence sources insist that CIA officers in Thailand work closely with the Thai military to ensure than none of the covert aid gets to the Khmer Rouge, and that the Thais themselves have set up stringent controls.

This modest covert-aid program is one sign of the Reagan administration's increasing willingness to offer support to groups fighting left-wing and communist governments in the Third World. Although the administration is still proceeding cautiously, many of its officials have begun to speak out about the need to help such insurgencies.

CIA Director William J. Casey, who made an unpublicized visit to the Thai-Cambodian border two months ago, told U.S. News & World Report in a recent interview, "Every U.S. president since Franklin Roosevelt has authorized support of rebels opposing an oppressive or illegitimate regime." He noted that Cambodia was being occupied by 170,000 Vietnamese troops.

In March, the Cambodian insurgents suffered a major defeat when Vietnamese forces overran their camps in Cambodia and forced them into Thailand. Thai and insurgent forces fought battles more than a mile inside Thailand when the Vietnamese spilled over the border.

Shultz is scheduled to visit a noncommunist resistance camp just inside Thailand "as a statement of support," a State Department official said. Shultz is on his way to the annual meeting of foreign ministers of the Association of Southeast Asian Nations (ASEAN), who have been asking the United States to get more directly involved in aiding the insurgents.

The United States has already become more involved in Thailand, where American military aid has tripled since the Vietnamese invasion of Cambodia, to nearly $100 million a year.

This year, Congress has moved to provide overt military support to the noncommunist opposition in Cambodia. Rep. Stephen J. Solarz (D-N.Y.) is pushing for $5 million in such aid, although the House has yet to act.

Reagan administration officials at first opposed overt military aid, but recently shifted and are supporting a version of the Solarz provision, already passed by the Senate, that lets the administration decide whether to supply economic or military aid. At this point, administration officials say, they see no reason to provide military aid.

After Vietnam invaded Cambodia in late 1978, sources said, the Carter administration began a small program to support Thailand's efforts to counter Vietnamese and Soviet influence. The funds were used for noncommunist insurgent leaders' travel expenses and for upkeep of resistance camps near the Thai-Cambodian border.

The Reagan program began as the United States and ASEAN were pressuring the noncommunist groups to make a coalition with the Khmer Rouge.

China -- which openly backs the Khmer Rouge -- and ASEAN both supply the insurgent groups with guns and ammunition. U.S. funds go only for "nonlethal" aid, sources said.

Some sources say this claim is misleading because the U.S. aid frees up other money that can be used to buy military equipment. They also say that the Khmer Rouge benefit indirectly because the U.S. money for the other two resistance groups makes the whole coalition stronger.

Despite the "nonlethal" label on the secret U.S. aid, one knowledgeable source said that a CIA logistics expert had traveled to Thailand to discuss the ammunition needs of the noncommunists, and CIA officers work closely with the Thai military men who advise the insurgents.

The only current overt U.S. aid is about $15 million a year in humanitarian aid to Cambodian refugees living at the Thai border.

Many officials acknowledge that the effort to strengthen the noncommunist resistance is a long shot. One informed source said that "of course, if the coalition wins, the Khmer Rouge will eat the others alive."

The Khmer Rouge are the strongest of the three factions fighting the Heng Samrin regime the Vietnamese installed in Phnom Penh. Pol Pot has about 35,000 fighters, according to State Department estimates. The noncommunist group headed by former prime minister Son Sann has about 15,000 troops, and the one led by former head of state Prince Norodom Sihanouk has perhaps 9,000.

Support in Congress for anticommunist insurgent groups everywhere has been growing, as recent votes indicate. The House approved sending "humanitarian" aid to the contras, or counterrevolutionaries, in Nicaragua. The Senate repealed a ban on aid to rebels in Angola. And Congress consistently has voted more covert aid to Afghan insurgents -- now about $250 million a year -- than the administration has requested.

A number of experienced U.S. intelligence officials who have worked in Southeast Asia are wary of new CIA involvements there. They say that maintaining meaningful control of both money and and covert operations is difficult if not impossible in a region where local intrigues magnify the dangers and uncertainties of all clandestine activities.

The most recent Reagan administration statement on overt aid came in a letter to the House Foreign Affairs Committee. It said the administration "welcomes the Solarz provision as an important signal to Hanoi regarding congressional and public attitudes toward Vietnam's illegal occupation of Cambodia and the threat it poses to its other neighbors."

Staff researcher Barbara Feinman contributed to this report. near the Thai-Cambodian border.

The Reagan program began as the United States and ASEAN were pressuring the noncommunist groups to make a coalition with the Khmer Rouge.

China -- which openly backs the Khmer Rouge -- and ASEAN both supply the insurgent groups with guns and ammunition. U.S. funds go only for "nonlethal" aid, sources said.

Some sources say this claim is misleading because the U.S. aid frees up other money that can be used to buy military equipment. They also say that the Khmer Rouge benefit indirectly because the U.S. money for the other two resistance groups makes the whole coalition stronger.

Despite the "nonlethal" label on the secret U.S. aid, one knowledgeable source said that a CIA logistics expert had traveled to Thailand to discuss the ammunition needs of the noncommunists, and CIA officers work closely with the Thai military men who advise the insurgents.

The only current overt U.S. aid is about $15 million a year in humanitarian aid to Cambodian refugees living at the Thai border.

Many officials acknowledge that the effort to strengthen the noncommunist resistance is a long shot. One informed source said that "of course, if the coalition wins, the Khmer Rouge will eat the others alive."

The Khmer Rouge are the strongest of the three factions fighting the Heng Samrin regime the Vietnamese installed in Phnom Penh. Pol Pot has about 35,000 fighters, according to State Department estimates. The noncommunist group headed by former prime minister Son Sann has about 15,000 troops, and the one led by former head of state Prince Norodom Sihanouk has perhaps 9,000.

Support in Congress for anticommunist insurgent groups everywhere has been growing, as recent votes indicate. The House approved sending "humanitarian" aid to the contras, or counterrevolutionaries, in Nicaragua. The Senate repealed a ban on aid to rebels in Angola. And Congress consistently has voted more covert aid to Afghan insurgents -- now about $250 million a year -- than the administration has requested.

A number of experienced U.S. intelligence officials who have worked in Southeast Asia are wary of new CIA involvements there. They say that maintaining meaningful control of both money and and covert operations is difficult if not impossible in a region where local intrigues magnify the dangers and uncertainties of all clandestine activities.

The most recent Reagan administration statement on overt aid came in a letter to the House Foreign Affairs Committee. It said the administration "welcomes the Solarz provision as an important signal to Hanoi regarding congressional and public attitudes toward Vietnam's illegal occupation of Cambodia and the threat it poses to its other neighbors."