White House spokesman Larry Speakes said yesterday that no U.S. government personnel were involved in Saturday's rebel attack in Nicaragua in which two Americans were killed when their helicopter was downed by Nicaraguan government forces.

An administration official traveling with President Reagan on a campaign swing through California said, "We had absolutely no association with this mission." Nicaragua said Sunday the helicopter and three airplanes had attacked a military training school 20 miles from the Honduran border.

The official, who declined to be named, described the two Americans as part of a group of seven U.S. citizens who had arrived in Honduras last week and "volunteered" to help U.S.-backed rebels in their fight to overthrow Nicaragua's ruling Sandinistas.

Nicaraguan Defense Minister Humberto Ortega said at a Managua press conference yesterday that the identification of the men as "volunteers" with no U.S. government ties was "a tale no one is going to believe." He called them "CIA mercenaries," special correspondent John Lantigua reported.

The men are believed to be the first Americans killed in Nicaragua since Reagan three years ago approved funding for a "secret war" against the Sandinistas. Although it is known that American intelligence personnel have been closely involved in training anti-Sandinista rebels and planning their missions, administration officials repeatedly have assured Congress that no U.S. personnel have been directly involved in the fighting.

Sen. Daniel Patrick Moynihan (D-N.Y.), vice chairman of the Senate Intelligence Committee, said yesterday that high-level CIA officials have told him on several occasions since the helicopter was shot down Saturday that the U.S. government was not involved "either indirectly or directly."

Reached by telephone yesterday in New York, Moynihan said a high-level CIA official told him there only "was a 2 percent possibility of a glitch," meaning that the U.S. might have been involved without the knowledge of top CIA officials. Moynihan said he told the official, "If there's been one, it's one hell of a glitch."

Neither the CIA nor the State Department has issued any detailed official statement about the incident, making Moynihan a principal public source of information. In announcing the deaths Saturday, the Sandinistas said three men were killed when the helicopter was shot down. They said they believed one of them was an American because of his height and light coloring.

But yesterday Ortega said that two of the three dead were Americans and cited Moynihan as his source. The Washington Post later was able to confirm that two of the men were U.S. citizens.

Moynihan said all of his information came from a ranking CIA official, whom he described as someone who "would have to know" what happened. But, he said, it was not clear from his conversations with the CIA official how the U.S. government knew that a group of "volunteers" had joined the rebels and why it did not do anything to stop them.

"That's a question that has to be answered," Moynihan said. He added that he believed the men had been recruited in New Orleans, possibly through Soldier of Fortune magazine, but he said that he did not believe they were recruited by the U.S. government.

Soldier of Fortune, a conservative Colorado-based publication that defines itself as a magazine for "professional adventurers," has published a number of reports of participation by its reporters and subscribers in fighting in Central America.

The U.S. official traveling with Reagan said the Americans killed in the crash "had been discouraged" by leaders of the Nicaraguan Democratic Force, known by its Spanish initials FDN, from undertaking the mission. But, he said, the main rebel leader, Adolfo Calero Portocarrero, was not in Central America when the Americans "talked the local contra rebel people into a mission involving a helicopter with these three people aboard."

NBC News reported that Calero, who is in Washington, said two men killed in the crash were Americans and were working for his organization.

Another spokesman for the Nicaraguan Democratic Force, the principal U.S.-backed rebel group, said in Tegucigalpa that the group will issue a statement today.

The State Department said yesterday that it was still "checking out" the report of the American deaths. It said the two dead Americans had not been identified. Meanwhile, Nicaraguan authorities issued a formal protest to U.S. officials and said they would deliver the bodies of the Americans to U.S. Embassy officials once the bodies are identified.

Congress has refused to continue funding the "secret war," and despite the continuation of fighting in Nicaragua both the Nicaraguan Democratic Force and the administration insist that the rebels have received no U.S. assistance since May.

"We still have some knowledge over what they are doing," the official said. "We don't have the kind of control and we are not able to exercise restraint as we would like to."

The official said there has been an increase in "well-meaning volunteers" offering their services. "So we have a situation of the contras continuing their activities with less control and less restraint," he added.

Special correspondent John Lantigua reported from Managua:

The official newspaper of the governing Sandinista National Liberation Front, Barricada, carried an account by Sandinista soldiers stationed near the base who said the helicopter participated in the attack, firing machine guns and rockets.

According to Barricada, the soldiers said that the helicopter was flying at a height of about 500 feet and that they knocked it down with fire from Soviet-made AK47 automatic rifles.

The soldiers, according to the newspaper, said they pulled one of the bodies from the wreckage before it was consumed by flames. The other two bodies, they said, were burned in the fire after the crash.

Journalists were invited to view the body of one of the Americans Sunday and take pictures, but the Sandinistas did not provide any identification documents.

Sandinista authorities revised the death toll in the attack on the base from five civilians to four, including three children.

Defense Minister Ortega said the downed helicopter was a Hughes 500 rather than an OH58 as originally reported.

A U.S. Embassy official said the embassy had been in touch with Nicaraguan Foreign Ministry officials who had said that if one or more of the dead men was identified as an American, the bodies would be delivered to embassy officials. The official said the embassy did not yet know for sure if the bodies were of Americans.

The Nicaraguans "said if the bodies are Americans, they'll be delivered to us, if they are Hondurans to Honduras, Salvadorans, etc.," said the official.

Foreign Minister Miguel D'Escoto said, "The U.S. government has made no attempt to claim the bodies or identify them. We believe that they are not about to claim the bodies."

He added that if the bodies are not claimed, the Nicaraguan government will give them "a decent burial."

In the official protest signed by acting foreign minister Jose Leon Talavera, the Nicaraguan government accused the United States of pursuing "a policy of state terrorism."

"The massive utilization of aircraft provided by the CIA to the mercenary bands is evidence of a greater level of development, an increase in the open war which the United States is fighting against my county," said Talavera.

"These acts reflect, once more, that the government of the United States has not yet taken seriously the process of dialogue with Nicaragua which is the responsible way to find peaceful solutions to the problems which the region is experiencing.

"The government of Nicaragua again urges the North American administration to abandon military means as a way to solve the controversy and at the same time, it urges the United States to assume responsibility in the process of dialogue with Nicaragua."