The United States, in a new bid to encourage negotiations on freeing the American hostages in Tehran, announced yesterday it is postponing indefinitely its long-threatened plan to impose economic sanctions against Iran.

The policy reversal came as part of the Carter administration's efforts to foster a conciliatory atmosphere in which Iran's new president, Abol Hassan Bani-Sadr, can move toward resolution of the 97-day-old crisis over the hostages.

State Department spokesman Hodding Carter said the decision to postpone sanctions was made within the last 48 hours at the highest levels of the administration. However, it had been known for two weeks that the administration was having second thoughts about the wisdom of its sanctions plan and was dragging its feet on issuing the regulations to put it into effect.

"The administration is holding the sanctions regulations in abeyance while diplomatic activities continue," Hodding Carter said. The decision, he added, "is aimed, we hope, at enhancing our diplomatic activities."

His announcement meant that the administration has dropped the last important vestige of the get-tough stance it took toward Iran in the early stages of the crisis. Instead, it now has shifted to a posture of trying to give Bani-Sadr, a moderate on the hostage issue, maneuvering room in which to assert his authority within Iran's divided power structure and isolate the militants holding the hostages.

The U.S. hope is that Bani-Sadr then will be able to begin talks on a "package deal" proposal, worked out in cooperation with the United Nations, that would see the hostages freed in exchange for an international commission investigating Iranian complaints about the alleged crimes of deposed shah Mohammad Reza Pahlavi.

Under this plan, the release of the hostages and the public announcement of the commission's findings would take place either simultaneously or in quick succession.

While the commission was pursuing its inquiries, the hostages either would remain in Iran under custody of a neutral organization such as the International Red Cross or, in a variation under discussion at the U.N., would be transferred temporarily to a third country. Sources said yesterday that Algeria has been mentioned as a third country.

Although the administration is hopeful about the prospects for this plan, U.S. officials repeatedly have stressed that so far Iran has not signaled its willingness to use the proposal as a basis for further talks.

That was reemphasized yesterday by Deputy Secretary of State Warren M. Chirstopher, who denied reports in Kuwaiti newspapers that a U.S.-Iranian deal on the hostages is imminent.

"I wouldn't say there's a deal in the making," Christopher said on NBC's "Today" program. He added that while "we have a somewhat promising situation and we're working hard on it, . . . I wouldn't want to put a time frame on it."

The U.S. shift on dealing with the Iranian crisis began last month following the Soviet Union's Dec. 27 invasion of Iran's neighbor Afghanistan. In the view of U.S. policy-makers, that action changed the situation from a test of wills with Iran's revolutionary zealots into a potential U.S.-Soviet competition for access and influence in the Persian Gulf, with its vital oil supplies.

Since then, the main thrust of U.S. policy has been on countering Soviet moves in that area. As part of that effort, U.S. officials said yesterday tentative plans have been made for Secretary of State Cyrus R. Vance to visit Bonn around Feb. 20 to discuss concerted Western strategy with the foreign ministers of West Germany, Britain, France, Italy and Canada.

President Carter met yesterday with Australian Prime Minister Malcolm Fraser, who reported on his recent talks with the leaders of Britain, France and West Germany on the allied response to the Soviet invasion. The president said he was pleased by Fraser's report, and announced that, in a further reaction to the Soviet invasion, a meeting of the foreign ministers of the United States, Australian and New Zealand scheduled for July was being moved up to later this month.

The United States last month began a campaign aimed at convincing Iranian leaders and public opinion that it is in their best interests to end the confrontation with the United States and turn their attention to potential threats from the Soviets.

But despite its efforts to be more conciliatory, the administration refused to drop the drive for economic sanctions that was launched in December and that Carter vowed to continue after the Soviets vetoed a sanctions resolution in the U.N. Security Council.

According to some administration sources, it was felt that dropping the sanctions move would damage Carter's credibility and expose him to charges of not doing enough to pressure Iran into freeing the hostages.

In a background briefing Jan. 17, a senior official and regulations formally spelling out the sanctions would be issued "within the next few days." In a television interview three days later, Carter vowed to go ahead with sanctions.

However, despite repeated administration denials that it was reconsidering the idea the regulations were never issued. Instead, in the wake of Bani-Sadr's election, some policymakers and major U.S. allies have argued that sanctions would be counterproductive.

As a practical matter, sanctions have been in effect since shortly after the outbreak of the crisis when the United States froze Iranian assets in U.S. banks. But, the argument was made, to codify them formally only would have the effect of strengthening anti-American forces within Iran seeking to block Bani-Sadr from settling the dispute.

Yesterday's announcement that sanctions are being postponed was welcome by Sen. Edward M. Kennedy (D-Mass.), Carter's major rival for the Democratic presidential nomination. In his Jan. 28 policy speech at Georgetown University, Kennedy had called for a package settlement similar to that now under consideration and had charged that sanctions might drive Iran toward the Soviets.