U.S. officials said yesterday that no Soviet fighter jets have been unloaded from a Soviet ship now docked in a Nicaraguan port and that they now believe the ship may not contain any of the MiG aircraft.

The officials cautioned that there still may be fighter jets aboard that will be unloaded later, or that because of U.S. pressure, they may never be unloaded.

But they said the chances that any MiGs are aboard grow dimmer as they see more vehicles, helicopters, small boats and other less-sophisticated equipment being unloaded at the northwestern port of Corinto.

As cargo was removed from the Bakuriani, administration officials began putting more emphasis on the dangers of a general arms buildup in Nicaragua, even if MiGs are not included. The Pentagon distributed a chart showing the growth of the Nicaraguan arsenal in the past five years, which one senior official said proved that the Central American country is "well on its way to becoming a second Cuba."

At the same time, officials at the Pentagon and State Department sought to quiet rumors that the United States was preparing to invade Nicaragua. While the United States would consider various options to "neutralize" any MiGs, a State Department official said, the idea of a general offensive is "nonsense."

Such statements did little to dampen invasion fever in Nicaragua, where the leftist Sandinista government mobilized its student militia to guard against what its leaders have warned might be imminent U.S. attack. Sonic booms over the country, presumably from U.S. spy planes, once again created some panic, according to Washington Post correspondent Robert J. McCartney. Details on Page A19.

Witness for Peace, an ecumenical religious group opposed to U.S. policies in Central America, sent 38 people to Corinto, "hopefully to prevent a U.S. invasion," according to spokesman Dennis Marker. He said that if U.S. Navy ships come within three miles of Nicaragua's shore the group "will go out to meet it with banners and nonviolent protest."

Pentagon spokesmen spent much of the day denying that they were planning anything of the kind. Their task was complicated by three military exercises -- a large-scale Army exercise in Georgia, a large naval exercise in the Caribbean and a smaller naval maneuver off the west coast of Central America -- which the spokesmen said repeatedly had been long in the planning and were not related to current tensions.

The Army exercise, dubbed Quick Thrust, involves a battalion each from the 82nd and 101st airborne divisions but mostly includes the heavily mechanized 24th Infantry Division, which Army officials said would be an unlikely choice for a Central American operation. The large naval exercise is more than 1,000 miles from Nicaragua, they said, and the smaller one near Corinto -- dubbed King's Guard -- involves only one U.S. ship.

State Department spokesman John Hughes said that any "hysteria" about a U.S. invasion stemming from the exercises is "misplaced."

"There is no U.S. threat to Nicaragua," he said.

The Pentagon would not confirm or deny the presence of SR71 Blackbird spy planes over Nicaragua. Officials said it is U.S. policy not to discuss "strategic reconnaissance."

The Reagan administration has been at odds with the Sandinistas almost since it was elected in 1980, alleging that the Nicaraguans are "exporting revolution" by aiding leftist rebels in El Salvador and threatening U.S. ally Honduras. During the past four years, the CIA has trained and armed a sizeable force of Nicaraguan rebels, mostly based in Honduras, who are seeking to overthrow the Sandinista government.

Tensions increased this week when U.S. officials said that the Bakuriani, which reached Corinto Wednesday, might contain MiG planes. U.S. officials repeatedly have warned the Nicaraguans not to import advanced fighters.

Officials said yesterday that evidence of the MiGs, based on satellite photographs of crates on the Soviet dock where the ship began its journey, was always circumstantial.

A senior administration official with President Reagan in California told reporters from the Associated Press and United Press International that it remains unclear whether the ship is carrying MiGs.

"We may never know," the official said, adding, "There may have been in the past few days a reconsideration of the wisdom of introducing MiGs into Nicaragua at this time."

But several officials said that, with or without MiGs, the arrival of the Bakuriani symbolizes an alarming increase in Soviet military shipments to the Sandinistas.

The senior official likened recent arms shipments, which officials said have included helicopters and antiaircraft radars, to the 1962 Cuban missile crisis that he said resulted in "a privileged position for the Cubans and guaranteed the continuing of Soviet sponsorship in Cuba."

"I think the political motive is quite high," he said. He said the Soviet Union has provided Nicaragua with a "full panoply of modern ground systems, including tanks, radar, patrol boats for the navy and the building of a modern, 10,000-foot runway for high performance jets."