A startled Carter administration clung to official hope yesterday that Ayatollah Ruhollah Khomeini's block-buster will not destroy the United Nations-sponsored arrangements for early release of the American hostages in Tehran.

Publicly, the administration maintained a policy of continuity, saying that the U.N. Commission of Inquiry "should go forward" with its mission and making clear that, in the American view, this mission encompasses a drive for release of the hostages.

Privately, administration officials displayed uncertainty, confusion and grave concern at a development that contradicts the U.S. understanding of Iran's posture in the hostage negotiations.

Despite the words of the Iranian communique containing Khomeini's personal pronouncement on the hostages, U.S. officials were not ready to concede that it means what it says. A State Department official characterized a literal interpretation of the announcement as "one extreme" in the spectrum of possible interpretations, saying that another "extreme" interpretation is that the statement is for political consumption in Iran.

The United States has never said publicly that dispatch of the U.N. commission is part of a deal to bring the hostages home within a few weeks, but official U.S. statements consistently linked the commission with the hostages. There have been strong hints that a U.N. commission member was right in saying that a "gentleman's agreement" covering both points had been made.

Khomeini's announcement, just before the New Hampshire primary, could leave President Carter in the position -- for at least six weeks -- of having reversed himself to allow the U.N. commission to hear Iran's passionate grievances at this stage without any clear sign or commitment that this will bring about the result he desires.

The United States first opposed formation of a U.N. commission until after the hostages are released, then opposed any operations by such a commission before the hostages are released. In a recent shift, it said no "effective working" by the commission could precede Iran's commitment to release.

In the present circumstances, the U.N. commission now in Tehran will have aired Iran's grievances and finished its work with no commitment to release. And if Khomeini's words are taken at face value, the new Iranian legislature seated in early April will discuss U.S. "concessions" to be demanded as the price for a still uncertain hostage release.

Carter could be left in this awkward if not untenable position as the early presidential primaries take place in New Hampshire, Massachusetts, Wisconsin and other states with a total of about one-third of the Democratic National Convention delegates. And there is no guarantee that the Iranian political process would bring about release of the hostages for many weeks after the legislature meets, if then.

While American officials refrained from saying so in the first flush of Khomeini's announcement, there were strong suggestions that they considered it, on its face, a breach of faith with both the United States and the United Nations. There were some hints that the commission members themselves may react when and if they are officially informed that their mission will not tangibly advance the hostages' release.

It was unclear here whether American officials had acquired a false impression of the release scenario from Iranian President Abol Hassan Bani-Sadr and his emissaries, or whether Bani-Sadr had been overruled at a crucial moment by Khomeini. Several additional possibilities were under study, including an analysis reaching a skeptical Washington through diplomatic channels that the Khomeini anouncement could be a mere tactical maneuver.

Among the surprises about the sudden announcement -- which came without advance warning to U.S. officialdom -- was the precise, almost lawyer-like drafting of the key passages regarding the hostages' future. This is an uncharacteristic style for Khomeini, raising the possibility that he had little or no hand in drafting it.

Some reports suggest Khomeini is sicker than generally known. Questions about his health and the present influences on him were heightened by a public statement attributed to his doctors, several hours after the hostage announcement, that visits to him are being barred for medical reasons.

As of last night, it could not be learned what the Carter administration will do or say if Khomeini's announcement is confirmed as the fixed policy of Iran. One alternative is to swallow pride and pursue the goal in restrained and patient fashion, despite the setback. Another alternative is to swing back toward a posture of threat and confrontation with Iran.

The official U.S. position yesterday, made public by State Department spokesman Hodding Carter, was to express continuing confidence in the U.N. commission and say nothing about the timetable for a hostage release.

"The commission has a mission. It is embarked on that mission . . . We believe it should go forward," said spokesman Carter. He said the purposes of the commission are two: "to hear Iran's grievances" and "to bring about an early end of the hostage crisis." On the latter point, he added. "That, of course, requires the release of all the hostages."

Noting that the commission was put together by U.N. Secretary General Kurt Waldheim, Carter said it is composed of "five prestigious individuals . . . committed to the purposes under which they took this job."

Carter said it is "very difficult" for the United States to read "what is meant" by the Khomeini statement, and the meaning could be debated forever. "We'll let the future speak for itself," he said, suggesting that unfolding events will fix its interpretation.

Asked whether the United States had been surprised by the statement, the spokesman said "after almost four months [since the hostages were seized Nov. 4] it is impossible to be surprised by anything that happens."

"The Iranians are continuing to violate the basic principles of international law and human rights by holding hostages for ransom," Carter said.

A State Department official conceded that the United States is "naturally concerned" by any development which interferes with quick release of all hostages. He added that officials had warned in the past of "ups and downs" in the drive for the hostages' safe return. The official's long face and grim demeanor left no doubt that this was among the "downs."