Whether President Reagan ever wins congressional approval of funds for rebels fighting Nicaragua's leftist Sandinista government, a private-sector campaign involving well-known conservatives is intensifying its efforts to keep the insurgents well-supplied.

The rebels continue to claim that they are well-funded, though it is impossible to establish precisely where the money is coming from. The two most prominent and active support groups identified so far are the World Anti-Communist League (WACL) and its U.S. chapter, the United States Council for World Freedom (USCWF). The head of both organizations is retired Army major general John K. Singlaub, who was ousted as chief of staff of U.S. forces in South Korea in 1977 when he publicly criticized President Jimmy Carter.

Singlaub apparently is an informal link among several other organizations raising money and political support for the "contra" rebels, whom they call "freedom fighters." The boards, donors and membership lists of these groups overlap, often reading like a "Who's Who" of the right. They say that in the wake of congressional refusal to provide U.S. aid to the rebels, it is up to private citizens to show U.S. support for democratic efforts worldwide.

Adolfo Calero, political chief of the Nicaraguan Democratic Force, the largest rebel group, said in an interview that "a substantial part" of his arms funds have come through Singlaub. He said his cash flow has improved recently, and estimated his total receipts at "close to $10 million," of which 40 percent is arms and the rest nonlethal help.

Many organizations send humanitarian aid to refugees in the area and try to avoid supplying any of the various armed groups. The Connecticut-based Americares Foundation, for example, dispatched $14 million in medical aid last year, mostly to El Salvador, and plans $20 million this year, distributed through Knights of Malta groups regionwide. About $3 million of that has gone to refugees in Honduras, where many of the families of Nicaraguan contras are living.

The president of the Americares Foundation, Robert C. Macauley, acknowledged that there is no way to guarantee that recipients are apolitical. Other aid donors, such as Singlaub, openly are helping the "contras" fight the Sandinistas.

In a recent interview, Singlaub said that he has raised almost $2 million outside the United States for arms for the Nicaraguan rebels, primarily through the World Anti-Communist League. (U.S. law bans fund raising inside U.S. borders for weapons to be sent overseas.) He and Calero said they were seeking military and financial help from WACL chapters in South America, noting that the chapters in Brazil and Argentina are large and active.

The humanitarian side of Singlaub's drive -- collecting medicine, food, clothing and other nonlethal aid -- has focused on domestic donors. This effort, he said, "has the support of the White House, the Pentagon and the Department of State."

Singlaub works actively for the Nicaraguan rebels' cause. Six weeks ago he was at a contra training camp with Calero offering advice and encouragement and promising to do more fund-raising. Within days, the general was seeking donations at a Palm Springs meeting of the conservative, 400-member Council for National Policy, made up of business, religious and political leaders, of which he is a board member.

Singlaub also is a board member of Western Goals, a conservative educational group founded by the late Rep. Larry McDonald (D-Ga.), and is on the advisory board of Refugee Relief International, an organization that has aided Salvadoran refugees that was established by editors of Soldier of Fortune magazine, a journal specializing in stories about mercenaries. Singlaub has said he has helped raise funds for Friends of the Americas, a Louisiana-based group chaired by Louisiana state Rep. Woody Jenkins, a conservative Democrat.

Jenkins said in an interview that his group has sent $1.5 million in medical aid to refugee groups in Honduras, including some Miskito Indians. His wife, Diane, a group director, said the aid includes 25,000 "shoeboxes" from private donors.

"They're like little CARE packages with a pound of beef, rice, soap, vitamins, candles and salt," she said, and sometimes include fishing lines, hooks and a mirror or photographs of the donors. She said they are worth $25 to $30 each.

Imposition of U.S. economic sanctions against Nicaragua, announced Wednesday by President Reagan, will lead to "thousands of people fleeing out of Nicaragua, and we hope to increase our efforts," especially on the Pacific Coast near the Nicaraguan border, Jenkins said.

Singlaub said the U.S. drive by USCWF and its allies is bringing in just under $500,000 a month, one third to one half of it from a group of wealthy Texas conservatives. They include Bert Hurlbut, president of First Texas Royalty and Exploration Co., prominent conservative donor Ellen St. John Garwood and Mr. and Mrs. John Howell of Howell Instruments. All confirmed that they had made donations.

Singlaub set up the U.S. Council for World Freedom in Phoenix, Ariz., in late 1981 with a loan of about $20,000 from Taiwan, according to retired Air Force lieutenant colonel Albert Koen, who was USCWF treasurer until May 1984. Koen said conservative Colorado businessman Joseph Coors was one of the group's few early backers and remains a staunch supporter.

The USCWF board includes several prominent conservatives: Retired lieutenant general Daniel O. Graham of High Frontier, the "Star Wars" lobby, as vice chairman; Anna Chennault, president of Transportation and Communications (TAC) International; John Fisher of the American Security Council; former U.S. representative John LeBoutillier (R-N.Y.), and Sammy Y. Jung, a Korean business consultant.

Hurlbut, who sits on the advisory board of the USCWF, said he heard about Singlaub through High Frontier while helping it raise funds. Since he joined in 1982, "the general and I have been working the fund-raising side of the street," Hurlbut said. He has traveled around the world with Singlaub and said the general is "treated like royalty by resistance forces everywhere."

Hurlbut has been heading the private drive in Texas with Singlaub. He said he and Mrs. Garwood have contributed more than $100,000, but emphasized that the money is used for medicine, food and clothing for the contras, their families and refugees.

"None of the funds from this country go for hardware. We've solicited funds elsewhere for that. The entire WACL board is trying to help out with arms," Hurlbut said.

The WACL chapter in France "has been very good in helping out" and the one in Britain "has been getting more involved," he said, referring to arms purchases. Chapters in Taiwan, South Korea and Saudi Arabia are among the most active and generous, each contributing more than $100,000 a year for WACL general operating purposes and more for emergencies or special projects, Hurlbut said. His statements could not be independently confirmed.

The World Anti-Communist League was formed in Taiwan in 1967 as an outgrowth of the Asian Peoples Anti-Communist League, a regional alliance against communism launched at the behest of Chiang Kai-shek after the Korean war. WACL board member and honorary chairman Dr. Ku Cheng-kang, head of the Taiwan chapter, has been a high level member of the ruling Nationalist Party in Taiwan for almost 50 years.

Hurlbut maintained that the Taiwan and South Korea chapters are sending $50,000 per month each to the contras. But Singlaub said that was "wishful thinking" and that Hurlbut was not in a position to know the figures.

Some WACL chapters have close ties to the Unification Church of the Rev. Sun Myung Moon. The Japanese chapter of WACL was founded in the late 1960s by Ryoichi Sasakawa, a wealthy conservative businessman who now heads the Japanese Shipbuilding Industry Foundation. He was jailed as a war criminal after World War II and subsequently helped start the Unification Church in Japan.

An arm of the Unification Church called Causa has run media seminars around Latin America for several years in the "cause" of anti-communism. Its director, retired general E. David Woellner, said the group has "set up our own channels of shipment and programs" to aid refugee groups in Honduras with food, clothing, toys, blankets and canvas for tents. He said the estimated $1 million in aid the group has sent since mid-1984 included a field kitchen, and that former U.S. ambassador John Negroponte had provided "cooperation."

"This program has been coordinated by the Honduran president's wife, the ambassador's wife and my wife," Woellner said. Former U.S. ambassador to Honduras Philip Sanchez is now head of Causa's U.S. branch, and its board of directors includes Daniel Graham of High Frontier and Lloyd Bucher, commander of the USS Pueblo when it was captured by North Korea.

WACL's most visible annual activities have been its conventions and its World Freedom Day rallies. Since the early 1970s, WACL conventions in Europe, Latin America and Asia have drawn delegates from 100 member countries and international groups. Recently they have included representatives from the anti-Castro Cuban terrorist group Alpha 66 and the far right Italian political party Italian Social Movement. The Italian terrorist group Ordine Novo, Croatian terrorist organizations and the Argentine AAA death squads also were represented, according to freelance writer Henrik Kruger, author of the book "The Great Heroin Coup."

Calero mentioned that he attended the WACL convention last September in San Diego and discussed contra needs with two WACL board members: Ku and Belgian Sen. Robert Close, a retired general who heads the European branch. "They said they were going to help and my understanding is that they have come through," Calero said.

Hurlbut said some USCWF board members have helped in innovative ways. Sammy Jung, the Korean consultant to American, Korean and Taiwanese firms, has obtained a large quantity of clothing for the contras at reduced rates. Hurlbut said he is trying to get a wealthy clothing manufacturer in Taiwan to provide similarly inexpensive clothing for the rebels, and said he has approached the Mormon church about providing seed packages in large quantities.

In the past month, Singlaub has made fund-raising trips to Fort Worth and Palm Springs, Fla., where he said he obtained about $100,000 in commitments from fellow members of the Council for National Policy. The 400 or so members of this group, headed until recently by Woody Jenkins, are religious, business and political conservatives including oil magnate Nelson Bunker Hunt, Christian Broadcasting Network chief Pat Robertson, singer Pat Boone and Robert J. Perry of Perry Homes.

An aide to Hunt confirmed that he has donated funds to aid Miskito Indians; Hurlbut said Perry was a contributor to refugee aid, but Perry could not be reached.

Much of Singlaub's 35-year military career involved classified programs and covert operations, starting with the Office of Strategic Services in World War II and then as a CIA station chief in Mukden, China. He was deputy CIA station chief in South Korea during the war there, and during the Vietnam war he ran a classified covert operation from 1966 to 1968 known as the Studies and Observation Group, or SOG. Using about 10,000 men, SOG ran secret raids, sabotage and psychological operations in North Vietnam, Cambodia and Laos.

His deputy during that program was Brig. Gen. Harry C. Aderholt, who now runs the Florida-based Air Commando Association that transports donated medical and other supplies to refugees, primarily in El Salvador.

Another transport organization, the Civilian Military Assistance Group, headed by Tom Posey and based in Alabama, claims more than 1,000 members nationwide and has sent several volunteer teams to fight with the contras. Two of its men were killed Sept. 1 when their helicopter was shot down over Nicaragua.

Last year, Singlaub headed a panel for Fred C. Ikle, the undersecretary of defense for policy, which recommended the use of more unconventional warfare tactics in Central America. Also last year, Singlaub set up a private center in Boulder, Colo., called the Institute for Regional and International Studies. He said it will "recruit people" with intelligence-gathering and psychological operations skills to train the Salvador police and perhaps the Nicaraguan rebels.

Singlaub is now planning this year's USCWF conference in Dallas this September. The final night's schedule is set: it will be a "Freedom Fighters Ball and Banquet" to support the contras.