An American captured by Nicaraguan troops appeared briefly at a press conference here late today, and the Sandinista military displayed credentials identifying him and the American copilot who died in a downed plane Sunday, as U.S. military advisers in El Salvador.

A second American, the pilot, also was killed, according to Nicaraguan officials.

The Nicaraguan government charged that the flight was operated by the CIA to resupply U.S.-backed rebels, known as contras, inside Nicaragua.

U.S. embassies in Central America denied the aircraft and its crew were linked to the U.S. government.

"My name is Eugene Hasenfus. I'm from Marinette, Wisconsin," said the tall red-headed man who was led out in front of a roomful of journalists by a Sandinista officer.

"I was captured yesterday in southern Nicaragua," Hasenfus said, in a statement lasting less than a minute. He was quickly steered away by Sandinista guards.

Hasenfus is the first American prisoner of war the leftist Sandinista government is known to have captured in five years of fighting against the contras.

Hasenfus, 35, was dressed in a mud-caked blue cotton work shirt, blue jeans and work boots. His face appeared bruised and swollen, but he stood and walked without aid. Deep sadness marked his features.

Lt. Col. Roberto Calderon, Sandinista Army commander in the southeastern jungle region, where he said the C123 military cargo aircraft was shot down Sunday afternoon, identified the deceased pilot as Capt. William J. Cooper.

The copilot was identified as Wallace Blaine Sawger, also killed in the crash. A fourth crew member was of Latin origin but could not be identified, Calderon said. The bodies, found inside the plane's smoking hull, have not yet been evacuated from the region, he said.

According to the Sandinista officer, Hasenfus said under questioning that he was a "kicker," the crew member assigned to push cargo out the plane's open rear door into the forests below. Hasenfus was said to have parachuted to safety when the plane was pierced by a portable Soviet-Bloc rocket fired by Sandinista infantrymen.

Hasenfus was captured at midday yesterday by Nicaraguan counterinsurgency forces as he fled through overgrowth about 30 miles north of the border with Costa Rica.

In the plane's charred and fractured wreckage, Nicaraguan troops found wallets they said belonged to the three Americans. One plastic-encased credential in Hasenfus' name bore his picture and indicated issue by the Salvadoran Air Force on July 20 of this year. It identified him as an "adviser" in the "USA" group at the Ilopango air base in San Salvador.

A similar reddish credential, its photo showing a man with a mustache, was in Sawger's name.

A third credential was issued to Cooper by a Miami-based air freight firm, Southern Air Transport. It was dated April 2, 1986, with a signature identified as that of personnel director Carl Holeva. The authenticity of the credentials could not be determined.

One wallet contained the business card of Capt. Humberto Villalta, an officer of the Salvadoran Navy. Another card belonged to P.J. Buechler of the State Department office that administered humanitarian aid for the contras during the past fiscal year.

In El Salvador, the U.S. Embassy said in a statement that Hasenfus "is not part of the U.S. military group here. He has no links with the U.S. Embassy. We don't know who he is."

The U.S. Embassy in Managua sent a diplomatic note to the Foreign Ministry requesting consular access to Hasenfus and additional information about the dead, according to Alberto Fernandez, the embassy spokesman, who said the embassy received no response. Fernandez said, "Neither the airplane nor its crew and cargo were financed by the U.S. government."

Sandinista troops removed from the wreckage about 70 new Soviet-made assault rifles, 100,000 rounds of rifle ammunition, about one dozen PG7 rockets and 150 pairs of combat boots, said Calderon, the Sandinista commander. Calderon said Nicaraguan intelligence had detected four other contra resupply flights from El Salvador since July. He asserted that the flights circle in from the Pacific Ocean over Costa Rica before looping north into Nicaragua.

[Earlier, Hasenfus told local journalists in San Carlos, near the crash site, that the flight began in Miami, picked him up in El Salvador, then went to Honduras, where it picked up a Nicaraguan, and entered Nicaraguan airspace from Costa Rica at a site called La Noca, The Associated Press reported.]

Calderon quoted Hasenfus as saying five contra resupply planes are parked at the Salvadoran Ilopango base. The downed plane was said to carry registry number C824, according to a log book Calderon held in his hands.

None of the three Americans appeared to be active-duty U.S. military personnel. Hasenfus reportedly said he served in the U.S. military in Vietnam until 1972 and had "continued to do the same work he did in Vietnam."

In Tegucigalpa, a contra spokesman said by telephone that all of his group's aircraft were accounted for undamaged "in their airports."

Carlos Icaza of the Nicaraguan Democratic Force (FDN), which with an estimated 14,000 men, is the largest contra force, said the group sometimes accepts aid, including air resupply of its field fighters, from "private volunteers."

But the contra leader said that in recent weeks no organization made any attempt to coordinate such assistance with the FDN.

Icaza said the CIA has not been involved in resupplying contra rebels with flights over Nicaragua since its mandate to provide aid was suspended in 1984. He said no U.S.-supported contra group currently has enough funds to obtain an airplane such as the C123.

The C123 is a twin-engine cargo craft that has been in production, with various models, since Fairchild introduced it in the 1950s. It has been a staple troop carrier for the U.S. Air Force and was in heavy use in Vietnam.