An international commission to investigate Iranian grievances has been selected by U.N. Secretary General Kurt Waldheim and approved by the United States, a U.N. official said today. But a delay in the expected approval by Iran has momentarily stalled formal announcement.

It was learned, nonetheless, that the five-member panel plans to gather in Geneva Tuesday and leave from there in a U.N.-chartered plane for Tehran, apparently to begin investigation at once of the alleged crimes of Iran's deposed shah.

Such preparations indicate that mediators here are nearing a firm agreement from Iranian officials to fix a time for releasing the estimated 50 Americans who have been hostage by Islamic militants in the U.S. Embassy in Tehran since Nov. 4.

After the announcement by U.N. officials here today that the panel had been approved by the United States, White House deputy press secretary Rex Granum said in Washington, "We don't have any reaction at this point."

While U.S. officials have been willing to allow the naming of a commission before gaining that commitment from Tehran, they have insisted on a more specific timetable for the release before a panel begins its work in Iran.

Iranian President Abol Hassan Bani-Sadr has broadly indicated he would gain release of the hostages after the inquiry begins, but he has drawn back from a fixed time to avoid a confrontation with the militant captors occupying the embassy.

The naming of a commission and defining of its manidate are seen as key elements in helping Bani-Sadr convince the Islamic militants of the "good faith" of the U.N.-inspired effort and the need to free their hostages.

In a telephone interview conducted yesterday with a Newsweek correspondent in Paris, Bani-Sadr said, "The liberation of the hostages depends on United States policy and a change in that policy."

He said, "The United States must take the initiative on three different points if it wants to change the political climate. It must condemn its past policy in Iran. It must promise not to interfere in our affairs in the future and it must promise it will not obstruct the pursuit of the shah, his entourage and other criminals for their treason and their other crimes."

Bani-Sadr said Iran's revolutionary leader, Ayatollah Ruhollah Khomeini, was "in complete agreement" with his position.

The timing of the hostages' release he said, "depends on the United States and whether the conditions are met."

The Reuter news agency reported from Tehran, however, that the militants holding the hostages appeared to be in no mood for compromise.

"By having the American spies in our hands we are in a position of initiative and power," they said in a statement last night, adding that the United State could not help but obey Iranian demands for the return of the shah and his wealth.

Reuter said Iran's Revolutionary Guards, the country's chief law enforcers, today also publicly expressed support for the militants and Khoemini's public position of two months ago that the hostages could not be released until the shah was extradited.

Bani-Sadr, by contrast, told Newsweek that "extradition is not a condition of the U.S. self-criticism."

By this evening, The Iranian president had yet to inform Waldheim that the panel members were acceptable. He was said to be seeking approval from Khomeini.

The delay appears to have more to do with the 8 1/2-hour time difference between New York and Tehran than any dispute over the panel, officials here said. In addition, Khomeini is often inaccessible because he is recovering from a heart ailment.

U.N. spokesman Francois Giuliani said the delay was "purely technical in nature." He said an official announcement would most likely be made Monday after Waldheim receives approval from Tehran.

Although it was not named publicly, the fact-finding commission awaiting approval in Tehran is understood to consist of:

Louis Pettiti, 64, the French author and former president of the Paris Bar Association, who was recently elected judge of the European Court of Human Rights.

Andres Aguilar, 54, Venezuela's former ambassador to the United States, who recently resigned as chairman of the Inter-American Human Rights Commission.

Mohamed Bedjaoui, 50, Algeria's ambassador to the United Nations and former ambassdor to France, who once served as secretary general of the Algerian government and minister of justice and keeper of seals.

Adib Daoudi, 56, a Syrian diplomat and former ambassador to European nations and India who serves as political adviser to President Hafez Assad.

Hector W. Jayewerdene, 63, the Queen's Counsel in Sri Landka and member of the U.N. subcommittee for protection of minorities. The brother of Sri Lanka's president, he is an attorney before his country's Supreme Court and president emeritus of the country's bar association.

Jayewerdene became a last-minute substitution for Abu Sayeed Choudhury, the chief judge and first president of Bangladesh, who was reported to be too ill to undertanke the mission in Iran.