President Idi Amin of Uganda has postponed a scheduled meeting with some 240 Americans at Entebbe Airport, reportedly causing the U.S. State Department to reconsider a plan to send a top diplomat to Kampala.

According to a broadcast monitored by the United States, another date for the session with American missionaries, teachers and others in the East African country will be announced later.

The State Department would not confirm that it had planned to send an American diplomat to Uganda in response to Amin's message inviting a U.S. representative to meet with him. Earlier reports said that Talcott Seelye, an experienced State Department troubleshooter who was posted to Lebanon during the civil war there, was to have gone to Uganda.

Amin told a small group of Americans already gathered at Entebbe yesterday that they were "brothers and sisters" to Ugandans and that he intended to honor them and all other Americans in the country.

Reports reaching Nairobi, Kenya, indicated that about 200 Britons had been asummoned along with the Americans to meet with Amin at Entebbe. British diplomats said they were trying to confirm the report.

The Ugandan announcement that the meeting was delayed came as a surprise. During the day, officials in Washington gave assurances that there was "no cause for alarm" about the scheduled meeting on Wednesday.

Seeking clarification, Secretary of State Cyrus R. Vance said the Ugandian charge d'affairs in Washington, Paul Cherubet, was being called in for consultations. He will confer with Philip C. Habib, the undersecretary for political affairs.

In an earlier broadcast, the Ugandan government quoted Amin as saying President Carter "was still new and young in office" and not quite familiar with African affairs. It said Amin had told a Saudi Arabian diplomat that Carter should have studied the situation carefully and found out "the true facts" before making any public comments.

The latest Ugandan announcement and a flurry of other reports added to the confusing array of speculation about Amin's motives and plans ince Friday when he ordered Americans not to leave the country before meeting with him.

The meeting was originally scheduled for Monday and then deplayed until Wednesday before this latest postponement. Presumably, the Americans still cannot leave Uganda before seeing Amin, which, in effect, would make them hostages of Amin. It is not known, however, whether any of the American residents of the country want to leave.

While insisting that he plans to honor the Americans, Amin also put his troops on 24-hour alert and warned the United States against attempting a commando raid on Entebbe like the Israelis mounted last summer to rescue about 100 hostages held by Palestinian guerrillas.

Amin, who has said he wants to thank Americans for their work in his country, was quoted as saying yesterday that the meeting was actually requested by the Americans.

Earlier, Uganda Radio, repeating its broadcast of the past two days about the meeting, omitted assurances that the Americans were in no danger.

Amin, however, was said to have told the Saudi Arabian diplomat that the Americans in Uganda were secure and well and tha Amin "has no problems at all" with them.

The newspaper Daily Nation, published in neighboring Kenya, which has extremely strained relations with with Uganda reported that the Americans in Uganda were being followed day and night by armed plainclothesmen.

A government source in Kampala said that more than 800 dancers and 1,000 chieftains and officials would be at Entebbe to entertain and honor the Americans. The source described the meeting as an expression of Amin's policy of "government by discussion" in which people were asked to voice problems directly to the leader.

Before the latest postponement, President Carter sent a message to Amin thanking him for his assurances that the lives of U.S. citizens in Uganda were not in danger. A White House spokesman said the message was sent through the West German embassy, which represents the United States in Uganda.

The Soviet news agency Tass accused the United States of "threatening" Uganda and following a policy of trying to intimidate "progressive African states."

The agency charged that Washington was using U.S. navy ships sailing off the East African coast as part of an overall strategy to suppress national liberation movements in Africa.