A senior administration official said yesterday that there is no clear evidence that Nicaragua has imported Soviet jet fighters, but there is "very credible evidence" that the Soviet Union is seeking to improve the "quality and quantity of arms" in that Central American nation.

The official said the United States is not looking for an excuse to invade Nicaragua and does not want to use force, as Nicaraguan leaders have charged. But he joined other officials who yesterday refused to rule out the use of force if the Soviets have delivered advanced MiG fighters to their Central American ally.

"We would take it very seriously," he said. "The U.S. isn't given to posturing vis-a-vis a challenge to our vital interests."

The latest round of tough statements from the Reagan administration and sounds of alarm from the ruling Sandinista junta in Nicaragua was stimulated by the arrival in the Nicaraguan port of Corinto of the Soviet arms-carrying ship Bakuriani on Wednesday. U.S. officials said there is "circumstantial evidence" that the ship may be carrying MiG21 jet aircraft.

U.S. officials said that evidence could not be independently confirmed or refuted today, in part because clouds in Nicaragua kept U.S. spy planes from obtaining useful photographs. U.S. officials said they would continue to monitor the ship carefully.

Soviet officials, while not publicly discussing the planes, privately have assured U.S. officials that they have not sent MiGs to Nicaragua, administration officials said yesterday. And the Nicaraguan government has denied heatedly that MiGs have been delivered or are on their way, saying that U.S. officials have raised the possibility as a pretext for military action.

A U.S. Navy frigate is steaming off the coast of Nicaragua not far from Corinto in international waters, Pentagon officials said. In addition, they said that a Honduran-Salvadoran naval exercise dubbed King's Guard, involving some U.S. personnel and the U.S. frigate, is beginning in the Gulf of Fonseca, north of Corinto.

A larger exercise involving the battleship Iowa and several other U.S. and allied ships is taking place in the Caribbean. But Pentagon officials said that that exercise is in the eastern Caribbean and western Atlantic Ocean, far from Nicaragua, and that neither exercise is related to the current level of tension in U.S.-Nicaraguan relations.

U.S. officials have repeatedly warned that Nicaragua would upset the balance of power by accepting MiGs. U.S.-backed forces in El Salvador and Honduras currently have significantly larger air forces than Nicaragua, but no country in the region has planes as advanced as MiG21s.

Nicaragua has said it is seeking aircraft to resist CIA-backed rebels seeking to overthrow its government. While officials in Managua have said they would prefer non-Soviet planes, they said they have a right to import MiGs if they can acquire them and choose to do so.

Sen. Daniel Patrick Moynihan (D-N.Y.), vice chairman of the Senate Select Committee on Intelligence, said the administration has decided what to do if MiGs are delivered but has not informed the Nicaraguan or Soviet governments. Asked whether he would rule out the use of force, he said, "I don't think it is possible to do that and be credible in our assertion, 'Don't do it.' "

The senior administration official said "we haven't reached that point" of deciding on a response if MiGs have been sent to Nicaragua. But he added, "There is no question that the escalation of the means to wage conflict beyond their borders in Nicaragua would have an important effect on our interests and our neighbors' interests."

Another senior official said that the administration has put its credibility on the line on the issue so that it could not tolerate the presence of MiGs. The second official said that the United States would have to deliver an ultimatum to the Soviets and Nicaraguans to remove the planes and, failing that, would have to take military action to destroy them.

Both officials stressed that the situation is not near that point yet, with the first saying that concern would be triggered when Washington can confirm "the presence of assembled aircraft in Nicaragua."

The official who spoke of "very credible evidence" of an escalating buildup said that the Soviets might send MiGs to Nicaragua, despite U.S. warnings, "to intimidate Nicaragua's neighbors, to weaken their resolve, their willingness to resist, their tenacity in negotiating for a stable peace -- and to make possible aggression."

"I think that the tactic of improving the quality of armaments residing in Nicaragua has a political purpose in probing the limits of U.S. tolerance," he said. He said that such a tactic would also be designed to give Nicaragua "leverage" in current peace talks with its neighbors, "upping the ante" in that peace process.