SAN CARLOS, NICARAGUA, JAN. 24 -- Sandinista troops last night shot down a DC6 cargo aircraft flying supplies to Nicaraguan rebels in the southern part of the country and captured a crewman who had parachuted.

The survivor who bailed out of the plane, Nicaraguan Alejandro Sanchez Herrera, 25, told reporters today that the flight originated on Swan Island, a Honduran territory where he said 30 Americans run a resupply operation for the U.S.-backed rebels known as contras. A Nicaraguan officer said that Sanchez, under interrogation, said the 30 were members of the CIA.

Sanchez said there were 10 crewmen on the plane -- all Nicaraguans except for the pilot, a Colombian whom he knew only as "Richard."

The Sandinista commander of the 5th military region, Lt. Col. Roberto Calderon, said the bodies of four crewmen had been found and that search parties were looking for the others. He said two of those on the plane had parachuted with supplies that were dropped before the aircraft was hit by two shoulder-fired SA7 missiles. The fate of three others on board was unknown, he said.

President Daniel Ortega called the flight a violation of Central American peace accords by "a party who didn't sign them," referring to the United States. The flight reportedly was one of the contras' first since the White House announced on Jan. 19 that President Reagan had ordered a resumption of resupply flights following a week-long suspension at the time of a regional peace summit in Costa Rica Jan. 15-16.

However, Sandinista military officers have charged that resupply flights went ahead during the supposed suspension, and the survivor, Sanchez, said he had been on such a flight on Jan. 17.

Reporters taken to the crash site in Soviet-made Sandinista military helicopters saw wreckage scattered in dense jungle about 20 miles east of San Carlos and a few miles north of the border with Costa Rica.

At a site where the bulk of the wreckage was found, the charred body of a man lay face up in what looked like the forward part of the fuselage. Nearby lay the charred and blackened remains of at least one other person. A small fire still burned under a fallen tree next to the body.

Parts of Soviet-design Kalashnikov and Belgian FAL automatic rifles lay on the ground, along with a U.S.-made yellow rubber life raft and other aircraft equipment. The life raft was marked, "Property of EAM rental program."

The World War II-vintage four-engine cargo plane was the first rebel resupply flight shot down by the Sandinistas since October 1986, when the downing of an American-manned plane in the same area resulted in the capture of Eugene Hasenfus, a cargo handler from Marinette, Wis. He was convicted of aiding the contras then pardoned after serving less than 90 days of a 30-year sentence.

A spokesman for the Nicaraguan rebels, Bosco Matamoros said in a telephone interview that 12 Nicaraguans had been aboard the plane, which had already made a drop of weapons and other equipment to rebels in southern Nicaragua when it went down. He identified the pilot as a contra named Donald Frixione, the copilot as Didier Guido and the flight engineer as Benjamin Sanchez.

Matamoros spoke on condition that the country where he was located not be identified. He said there were no Americans, U.S. government employees or high-ranking contra commanders on board. He said contras in the area also had been dispatched to hunt for survivors.

Sanchez said he had been a cargo handler, or "kicker," on eight contra resupply flights originating on Swan Island since Dec. 18. He added that Americans ran the resupply operation at a base guarded by Honduran soldiers, but he said no Americans had been on the flights he made.

Sanchez, appearing frightened but unhurt as he briefly answered questions in front of reporters at a Sandinista military base here, said he was captured last night by militiamen near the resettlement camp of Los Chiles, about three miles from the crash site.

After the appearance, he was hustled away with his hands manacled behind his back and driven off in an Army jeep.

Lt. Col. Calderon said Sanchez had told interrogators that a contra explosives expert and a shoulder-fired missile specialist parachuted from the plane when it dropped nearly 9,000 pounds of arms, ammunition, food and clothing to contras of the Jorge Salazar II Regional Command. Calderon said about 1,500 contras were operating in his military region in southeastern Nicaragua and that his forces had detected 15 resupply flights in the zone so far this month.

Calderon said the downed flight was one of the "weapons of the CIA" in the contra war against the Sandinista government. He displayed documents that he said showed U.S. involvement in the resupply operation. One was an old-looking 8-by-10 photograph of four men posing in front of a U.S. Air Force plane.

Sanchez, said Calderon, had idenified one of the men -- who were wearing airmen's uniforms -- as the leader of resupply operation on Swan Island, a man he knew as "Johnson." The picture was printed by a photo studio in Texas. There was no explanation of why the photo was carried aboard the plane or to whom it belonged.

Also displayed were two U.S. Air Force survival manuals, some Honduran money, a document that allowed Sanchez to travel to the Honduran capital of Tegucigalpa, and assorted communications papers, maps and three other photographs -- two of young girls, possibly the daughters of one of the crewmen, and one of an unidentified man and woman.

A Sandinista official at the resettlement camp, whose militiamen captured Sanchez, said he saw the plane zigzag and lose altitude as it flew east after being hit by the ground-to-air missiles.

The official, the director of an agricultural cooperative called "The Great Lenin," said many of the 3,000 persons in the resettlement camp have relatives among the contras.