United Nations Secretary General Kurt Waldheim called last night for an urgent Security Council meeting to deal with the "crisis situation" between the United States and Iran, describing it as the most serious threat to world peace since the Cuban missile crisis of 1962.

The Carter administration, which previously had opposed a formal U.N. session, approved Waldheim's initiative in the belief that it will produce a strong new international appeal for the release of the 49 Americans held hostage in the U.S. embassy in Tehran.

A State Department official who briefed reporters said there is no indication that Iran will heed a new Security Council plea by freeing the Americans, held since Nov. 4, But he called Waldheim's action "one more step along the diplomatic road" and said that every avenue should be explored in a deteriorating situation.

President Carter, returning yesterday afternoon to the White House from an eight-day Thanksgiving stay at Camp David, said he is "not particularly" optimistic that a new U.N. appeal can secure the hostages' release but said that "we're trying in every way" to achieve that result.

One result of the Security Council session, which could take place as early as today folowing consultations among members this morning, will be to give Iran a forum for its attacks on the United States and on the deposed shah, Mohammad Reza Pahlavi. As a party of interest, the Iranian delegate will be able to speak on such points during the debate.

The previous U.S. position, taken to the 14 other Security Council members by Secretary of State Cyrus R. Vance in person two weeks ago, rejected an airing of Iran's grievances in the world body until that country agreed to release the hostages.

At the State Department yesterday, the shift in Washington's position was ascribed to "new factors" including "increasing danger to the hostages" and Iran's threat to try them as spies.

The United States is certain that the debate will end with adoption of a resolution calling for the immediate release of the hostages, according to State Department officials. Another part of the resolution may ask that the United States and Iran reconcile their differences in a peaceful fashion, officials said.

Waldheim's statement to reporters at the United Nations, after a day of urgent discussions with a variety of governments, indicated that White House suggestions of the use of military force, in case the hostages are harmed, was an important factor in the sharply rising international concern that led him to take action.

"In my opinion, tension has now escalated to such an extent that a threat has been created not only to the peace and stability of the region but to the entire world," the secretary general declared.

Waldheim asked for the Security Council meeting on his own initiative under a provision of the United Nations charter that has been used only once before, by then Secretary General Dag Hammarskjold in the 1960 Congo crisis. The provision allows the U.N.'s top official to take questions directly to the Security Council in case of a threat to "international peace and security."

In his letter to Council President Sergio Palacios de Vizzio of Bolivia, Waldheim said:

"The government of the U.S. is deeply disturbed at the seizure of its embassy in Tehran and the detention of its diplomatic personnel, in violation of the relevant international conventions.

"The government of Iran seeks redress for injustices and abuse of human rights which, in its view, were committed by the previous regime.

"The international community is increasingly concerned that the dangerous level of tension between these two countries threatens peace and stability in the region and could have disastrous consequences for the world."

Security Council President Palacios issued a statement Nov. 9 asking "in the strongest terms" that the hostages be released by Iran without delay in accordance with international practice. This statement was authorized by a negotiated "consensus" of the 15-member council, without a formal meeting or vote.

The Soviet Union, which has a veto in the Security Council, is not expected to object to a new and formal demand that the Americans be freed, according to State Department sources.

There have been recurrent suggestions in the statements of some Iranians, notably acting Foreign Minister Abol Hassan Bani-Sadr, that U.S. or international declarations ascribing guilt to the deposed shah could play a role in the release of the U.S. hostages.

There was every indication that the U.S. refusal to provide such a statement remains unchanged. President Carter told reporters the United States will not consider condemning the shah in order to obtain the release of the hostages. State Department officials said the U.S. statement to the Security Council which probably will be delivered by Ambassador Donald F. McHenry, will not contain an attack on the shah.

Sources close to the New York hospital where Pahlavi is being treated for cancer and gallstone disorders said there was no indication last night of an imminent move by the former Iranian leader to return to Mexico, despite some reports to the contrary.

U.S. officials continue to express uncertainty about the effect on the hostages of such shah-related developments as his departure from the United States, an investigation of his rule by an internationally sponsored body or an official condumnatin of his alleged misdeeds.

The continuing puzzle of who speaks for Iran and with what authority is a major factor in the uncertainty. The difficulty was dramatized anew when Iran's official news agency announced yesterday that Bani-Sadr would fly to New York today to take part in the Security Council debate -- and then announced a short time later that the official had postponed the trip for at least a week.

It was unclear whether Bani-Sadr planned to come to New York but was overruled, or whether he changed his mind for some reason at the last minute.