The United States yesterday gave Algerian intermediaries a reformulated version of its proposals for freeing the American hostages in Iran, and let it be known that if the Iranians don't accept the plan by Jan. 20, when President Carter leaves office, it may no longer be available.

U.S. officials described the latest plan, worked out during four days of talks here with the Algerian envoys, as being more detailed than previous U.S. proposals and containing some "new twists." But the officials said its basic substance does not go beyond past statements of what the Carter administration is prepared to do to meet Iran's conditions for releasing the hostages.

The previous U.S. offer, which the State Department made public Sunday, hit a snag when Iran demanded $24 billion in guarantees that the United States would keep its promises -- a demand that is legally and politically impossible for the United States to meet. According to the officials, the main thrust of the reformulated U.S. plan is to underscore more clearly to Iran the assurances and guarantees already contained in the American proposals.

That was hinted at by Deputy Secretary of State Warren M. Christopher, who said that the U.S. position has been reformulated "in a way that will enable prompt resolution of this matter if the Iranians are willing and able to do so." But, Christopher added, while the Carter administration still hopes to bring the 423-day-old hostage impasse "to a successful conclusion on our watch," it is prepared to turn the matter over to President-elect Ronald Reagan on Jan. 20.

U.S. officials said Christopher's remark was intended to remind the Iranians that the U.S. offer technically is on the table only until that time, and that Reagan then would be free to withdraw it for another, possibly tougher approach to dealing with the situation. In the talks with the Algerian intermediaries, the officials said, the United States put special emphasis on the fact that time is running out on what amounts to the Carter administration's final offer.

In fact, the officials said they were skeptical that enough time remains to reach a deal before the presidency changes hands -- a situation that was acknowledged publicly yesterday by White House press secretary Jody Powell, who said, "The chances are slim." But, as one U.S. official also noted, "It's a longshot, but it has a chance of being a good shot."

A crucial factor in determining the fate of the new American proposal is the extent to which the Algerian intermediaries may be willing to depart from their "honest broker" role to recommend it or even guarantee its performance to the Iranians.

This took on new importance yesterday, when Bezhad Nabavi, who heads the negotiating committee of the Iranian government, told a Tehran news conference that guarantees acceptable to the Algerians "will also be acceptable to us."

The Algerian intermediaries, including Redha Malek, ambassador to Washington, and Abdel Karim Gheraieb, ambassador to Tehran, will now take the latest U.S. plan to Algiers for several days of discussion with higher officials there.

U.S. officials said the Algerians so far have not accepted a more active role in advancing the U.S. plan, but suggested that this remains an open question that may be decided in the Algiers talks.

With less than three weeks to go, Washington officials said that Iran would have to accept nearly all the elements of the latest U.S. plan in pretty great detail in order for a settlement of the hostage issue to be reached before the Carter administration leaves office.

There were no predictions among those most clearly informed that Iran will be able to resolve its internal disputes quickly and cleanly enough for this to happen, particularly because the U.S. proposals depart in major fashion from Iran's publicized "final answers."

But one official said that settlements that seem impossible can sometimes be reached, when all sides are willing, in the final days before negotiating deadlines in labor and other disputes.

In documents made public Sunday, the administration revealed what it was willing to do to meet the conditions set by the Iranian parliament for the hostages' release. The U.S. offer, conveyed to Tehran earlier, included the immediate transfer of $2.5 billion in blocked Iranian funds, binding arbitration by an international commission of claims by American companies and individuals against Iran, and a freeze on the assets in this country of the late shah Mohammad Reza Pahlavi and his family.

In setting out its newest plan, the officials said, the United States has made clear that it will not put up any cash guarantees to cover compliance with the various Iranian conditions and has said, in particular, that it will not guarantee in any way the outcome of any effort to recover the shah's assets through action in the U.S. courts.

That means, in effect, that Iran will have to back down from its demand that the United States assume responsibility for ascertaining the value of the shah's assets in this country and guarantee their return to Iran.

The U.S. position, as restated in the latest American plan, is that U.S. law requires Iran to pursue its claims in the federal courts and that the executive branch can do nothing beyond helping to identify which of the shah's assets are here and attempt to block their removal pending a court disposition.

In response to reports that the United States now has proposed putting some of Iran's frozen assets in an escrow account under the control of Algeria or another neutral country, the officials said the possibility of using such a device has always been an option under American proposals.

In fact, they added, it seems like a logical device because of U.S. insistence that none of the frozen Iranian assets be returned to Iranian control until the hostages are free.

But the officials continued, while some kind of escrow arrangement has not been ruled out, there first has to be agreement between Washington and Tehran on details of a mechanism for arbitrating the American claims and an explicit statement by Iran that it will accept and abide by the findings of the arbitration panel.