A Soviet cargo ship that U.S. officials said may be carrying Soviet fighter planes docked in the Nicaraguan port of Corinto yesterday, but Nicaraguan officials vehemently denied that any planes were aboard.

The Nicaraguans charged that the U.S. statements were designed "to prepare the climate for direct military attacks against our territory."

U.S. officials in Washington said they suspected that the ship, described here as the arms-carrier Bakuriana, may be carrying about a dozen MiG21 jets. They said their suspicions are based on satellite photographs that showed distinctive packing crates containing the MiGs on a Soviet dock next to the Bakuriana when the ship was at a Black Sea port several weeks ago.

U.S. diplomats expressed concern to Soviet officials here and in Moscow on Tuesday, administration officials said. But U.S. officials also cautioned that they are far from certain about the ship's cargo, and there appeared to be some confusion within the administration about the strength of the evidence.

Miguel D'Escoto, Nicaraguan foreign minister, said in a news conference in Managua yesterday that the ship's cargo "may well be" military. But he said that "it is a lie" to suggest that warplanes of any kind have been delivered or are on their way.

Two non-U.S. diplomats in Managua said the government's denials were the strongest ever issued on the subject, and U.S. officials there said the denials should be "considered carefully."

The Reagan administration has warned the leftist Sandinista government in Nicaragua since 1981 that it would not tolerate the presence of advanced fighter planes in that Central American country, saying such planes would upset the regional balance of power.

Nicaraguan officials have said they are seeking new aircraft to defend against attacks by CIA-backed rebels known as counterrevolutionaries or contras. The officials have said they prefer to obtain planes from countries other than the Soviet Union, but efforts so far have been unsuccessful.

President Reagan, in a news conference yesterday, said he could not comment "on any plans on what we might do" if Nicaragua imports MiGs.

"We ourselves have been alerted and are surveilling that ship, but we cannot definitely identify that they have MiGs on there, or planes of any kind," he said. "We're keeping a careful watch. As I say, I'm not going to comment on what might follow, or what our procedure might be."

The three-man governing junta in Nicaragua, "categorically" denied that any warplanes have arrived or are en route, and suggested in a communique that the United States is trying to create a warlike atmosphere with its accusations.

"The government of Nicaragua manifests categorically that it is false that any boat of any nationality is transporting combat airplanes toward Nicaraguan ports, or that combat airplanes have been unloaded," the communique said.

D'Escoto also charged that a U.S. warship steamed closer than 12 miles from Corinto yesterday morning after the Soviet ship docked and that U.S. aircraft flew overhead all day.

The Sandinista Air Force yesterday flew about 20 journalists over what it said was a U.S. warship about 15 miles west of Corinto. About the size of a frigate, the gray ship showed no flag and had the numbers "1080" in white on the bow.

Pentagon officials acknowledged that one U.S. frigate is off the west coast of Nicaragua, but denied that it violated Nicaragua's territorial limits. They said that U.S. planes shadowed the Soviet ship for much of its journey from the Black Sea.

Secretary of State George P. Shultz said in an interview he does not believe the United States is approaching a crisis in Central America, adding, "But we will watch and see if high performance aircraft go in there."

D'Escoto declined to deny that a separate supply ship of Bulgarian registry that arrived at the Caribbean port of El Bluff about two weeks ago had delivered military helicopters, as high-ranking diplomatic sources in Managua have said. U.S. officials in Washington said yesterday that the helicopters, which also have been seen only in their crates, are advanced Mi24 Hind gunships.

"Any defense materiel that we might be getting, or are getting, or will continue to get . . .is only an exclusively defensive type of military equipment," D'Escoto said.

Administration officials said that Michael Armacost, the third-ranking official in the State Department, communicated U.S. concerns at the Soviet Embassy here Tuesday and that Ambassador Arthur Hartman expressed the same concerns to the Soviet Foreign Ministry in Moscow.

U.S. officials have expressed alarm several times in the past that MiGs might be on their way to Nicaragua. Officials have said that crates containing the planes have been spotted in Cuba, that Nicaraguan pilots were being trained in Bulgaria and that long runways have been built in Nicaragua.

In this instance, U.S. officials said they had pictures of about a dozen crates on a dock in a Soviet port near Odessa. The Bakuriana was docked nearby, and the next time officials could get clear pictures of the dock the crates were gone.