WASHINGTON -- A "friendly foreign government" acting on behalf of the United States financed the flight of a cargo plane shot down over Nicaragua on Sunday with a load of weapons for anti-Sandinista rebels, an administration official and a well- briefed contra source said Tuesday.

They did not identify the friendly government but said the plane took off from a military air base in El Salvador and the government of President Jose Napoleon Duarte either was involved or at least knew of the flight in advance.

Eugene Hasenfus of Marinette, Wis., an American who was captured by Nicaraguan troops and appeared to be the only survivor of the plane's four-man crew, is an ex-Marine who once flew for a CIA-operated airline in Vietnam, according to his brother William in Oshkosh, Wis. One of the three crewmen who died in the crash, William J. Cooper, also flew for the CIA in Vietnam, the brother said.

Officials at the White House, the State Department, the Pentagon and the CIA denied Tuesday that Hasenfus, the downed plane or the other crewmen aboard it were connected to the U.S. government.

Secretary of State George Shultz said the aircraft, which Sandinista officials said was a C-123 propeller-driven cargo plane, "was, for all we know, a plane hired by private people, apparently some of them American. . . . They had no connection with the U.S. government at all."

The administration and contra sources said the plane was carrying supplies bought by the contras on credit, in anticipation of the $100 million aid package that is still pending in Congress. The sources said international arms dealers have resumed shipments to the rebels in recent weeks, charging the equipment against an open account that will be paid out of the $100 million fund.

The sources said the flight was one of several flights that have originated from Ilopango air base outside El Salvador's capital to supply the contras since the government of neighboring Honduras began blocking the delivery of aid to the rebels through its territory last year.

Two aviation sources in the United States said Tuesday that the type of plane used, a C-123, is not owned by any commercial firm in the United States but that several of them have been transferred by the Air Force to allied governments in Central America and Asia in the last two years.

The administration official consulted for this article said Hasenfus, the captured American, was aboard the plane as a "load specialist" in charge of readying the supplies for airdrops to the contras.

He had been chosen because of his previous experience in the Marines as a parachute rigger and expert in airdrops with Air America, a so-called CIA propietary company that flew secret cargo missions in Indochina during the Vietnam War.

William Hasenfus said his brother Eugene served in the Marines from 1960 to 1965 as an air delivery specialist for equipment drops. He said Eugene was stationed at Camp Pendleton, Calif., and never served in combat. Official Pentagon records confirm Hasenfus' Marine service record. William Hasenfus said his brother went to work for Air America after his stint in the Marines.

PARACHUTED TO SAFETY

The Nicaraguan Defense Ministry said in Managua on Monday that Hasenfus parachuted to safety when the plane was shot down by Sandinista army troops with a Soviet-made ground-to-air missile, about 35 miles north of the border with Costa Rica.