The United States expressed the "strongest concern" yesterday about the safety of Americans in Uganda after Uganda President Idi Amin claimed that the United States planned to invade his country.

Amin's instructions to Americans to appear before him on Monday with lists of their property caused the Carter administration to begin crisis-style planning. Officials were unsure if they actually had a crisis on their hands, but said Amin's "total unpredictability" made precautions necessary.

At the end of a confusing sequence yesterday in which Uganda spokesmen claimed "the American ministerpreted" what was happening, the mood of incipient crisis diminished in Washington.

A spokesman in Uganda, quoted by Uganda Radio, said Amin only wanted "to thank all Americans for the excellent work they have been doing in Uganda since the closure of the American embassy in Uganda in 1973 during the Arab-Israeli war."

Presidential press secretary Jody Powell read the text of that broadcast to reporters and then said, "We consider this broadcast helpful in allaying concern."

Administration sources said they beleived Amin decided, at least for the present, "to cool it" after arousing alarm earlier in the day with his message to Americans in Uganda, and after sending a rambling, 5,000-word telegram to President Carter. In the telegram, Amin charged that Carter was being given false information in a "dirty campaign against Uganda" by "the United States, Britain and Israel," stimulted by "pressure from the Zionists."

President Carter on Wednesday said that Uganda's actions "have disgusted the entire civilized word," and he deplored "the horrible murders that apparently are taking place in that country."

The alarms in Washington were set off early yesterday by reports that Amin ordered Americans to assemble on Monday. A task force of diplomatic, military and intelligence officers was assembled at the Senate Department to consider means of ensuring their safety.

Tension built up in Washington throughout the day, starting with a White House annoucement that "we are reviewing the situation with concern and monitoring it closely," with information relayed through the West german government, which represents U.S. diplomatic interest in Uganda.

President Carter conferred with Secretary of State Cyrus R. Vance, and other top advisers, as well as with United Nations Secretary General Kurt Waldheim, who said later that he told the President "that if he considered it useful and helpful I would be ready" to go to Uganda to intercede with Amin for the Americans there.

The senior Uganda diplomat in Washington, Paul C. Cheplwurui was summoned to the State Department and told by William E. Schaufele Jr., assistant secretary for African affairs, of the "strongest U.S. concern" about the implications of the Uganda annoucement.

It was the second trip by Chepkwurui to the State Department in two days. On Thursday, a spokeman said, Chepkwurui was told that Uganda's "false public allegations of U.S. complcity to overthrow President Amin might have the effect of inflaming public opinion against Americans there."

Chepkwurui told reporters yesterday as official U.S. concern intensified that "there is no cause for alarm" about the Americans in Uganda.

He said, "President Amin wishes to inform them that they would not be bothered or harrassed. He wants them to know that he has appreciated the work that they have done and he wants them to continue. But if they want to leave, they can go, and they won't be kicked out earlier."

The same theme was reiterated by Uganda embassy spokesmen throughout the day. At the same time, however, Amin's accusatory message to Carter was being transmitted. It included the renewed claim that "5,000 American Marines . . . are supposed to come and rescue 250 American missionnnaires in Uganda," but Uganda has the strength to crush any invaders."

Adminstration spokesmen described the invasion claim as "totally false" and "absurd." But administration sources acknowledged they were quite concerned about what action Amin might take, as one put it, "however irresponsible that might be."

President Carter, spokesman Powell said at midday, "will do whatever he feels is both necessary and proper to protect American lives anywhere."

Officials refused to discuss what forms of military rescue might be attempted, if necessary. Inevitably, speculation arose about whether the united states was considering anything comparable to the daring Israeli commando raid at Uganda's Entebbe airport last July, to rescue Israeli hostages hijacked by Palestinian guerrilas.