The Carter administration, noting that "nothing has happened yet," reacted with extreme caution yesterday to reports from Tehran that the militants holding the 50 American hostages will turn them over to Iran's Revolutionary Council.

U.S. Sources said privately the latest development had caught Washington by surprise at a moment when the administration was expected the United Nations commission visiting Tehran to leave without having made any progress toward the hostages' release.

On the surface, the officials added, the events in Tehran seemed an optimistic signal that the long crisis might finally be coming to a resolution. oBut, they stressed, the United States still doesn't have a clear picture of what is hapapening there and can only wait for further developments to clarify the situation.

For that reason, the officials said, the United States is waiting to see what the Revolutionary Council does today to follow up on its announced intention to take custody of the hostages who have been held in the U.S. Embassy compound since its seizure by the militants on Nov. 4.

Recalling the many past instances in the crisis when hopeful signals from Tehran have turned out to be illusory, the officials warned that some unexpected new reversal could come up suddenly and throw the situation into renewed confusion.

In particular, the officials said, there is concern here that the power struggle pitting the militants against Iranian President Abol Hassan Bani-Sadr and the Revolutionary Council may not be over. There is always the chance, the officials noted, that the militants may resort to some last minute maneuver aimed at neutralizing the pressure being exerted against them.

In addition, the officials pointed out, while transfer of the captives to the council would be a very encouraging development, it would not necessarily mean the automatic or even speedy exit of the hostages from Iran.

There still is no sign whether Iran intends to stick with the ruling by Ayatollah Ruhollah Khomeini, Iran's most powerful figure, that the hostages' fate will be decided by the new Iranian parliament, which isn't expected to begin meeting before April.

Even if that decision no longer stands, U.S. sources noted, Bani-Sadr, despite his obvious desire to end the crisis, may still have to engage in some delicate maneuvering between the diverse elements of the Iranian power structure before he can let the captives go without seriously damaging his power base.

In this respect, the sources said, Bani-Sadr is likely to continue insisting that the United States make some sort of "self-criticism" about its past activities in Iran and promise not to interfere in Iran's affairs in the future or impede Iranian efforts to obtain the return of deposed shah Mohammad Reza Pahlavi.Satisfying those demands, the sources said, is certain to mean further delicate negotiations.

For these reasons, the administration fell back on a wait-and-see stance that was summarized publicly by State Department spokesman Hodding Carter: "I am unaware of any physical change in the control of the hostages. cI am unable to give you any expectations or projections."

Or, as one source put it privately: "At the moment, the best we can say is that we may be beginning to see the light at the beginning of the tunnel. But it's all happened so fast we're not eveen sure that the tunnel is really there."

In fact, administration sources admitted, when the first word of the militants' willingness to give up their captives reached Washington Wednesday night, the State Department was expecting the U.N. commission's departure from Iran.

Under the supervision of Secretary of State Cyrus R. Vance, the sources said, the department was preparing a public statement expressing regret over the commission's failure to make headway on the hostage issue and signaling a return to the get-tough tactic of asking other countries to join the United States in putting economic pressure on Iran.

Then, the sources continued, Vance was given the news during President Carter's White House dinner for West German Chancellor Helmut Schmidt. After taking the president aside for a private, 15-minute talk, Vance left the White House abruptly to go to the State Department and monitor developments.

In their tentative assessments of the situation, U.S. officials said the events appeared to back up their belief that the hitches that had stymied the commission's activities resulted from an internal struggle between the militants and Iranian moderates led by Bani-Sadr.

In agreeing to appointment of the five-member commission to hear Iranian grievances against the shah, the United States thought it had a commitment from Bani-Sadr that the hostages would be freed after the commission completed its work. But once the commission got to Tehran, it ran into strong resistance from the militants, who said they would respond only to a direct order from Khomeini.

U.S. sources said they believe the impasse was broken largely by Iranian fears that the commission, whose members are considered sympathetic to Iran, would leave in anger and charge Iranian authorities with breaking their word.