President Carter, calling the Iranian government's decision to take custody of the American hostages in Tehran "a positive step," said yesterday he will defer "for the time being" U.S. moves to impose new economic sanctions against Iran.

But, in a pointed reference to the fact that Iranian authorities still have not carried out the decision announced by President Abol Hassan Bani-Sadr, Carter warned that there are limits to his patience.

He spoke against a background of conflicting statements from Tehran that continued through the day, and made clear that the tug of war between the Iranian government and the militants holding the hostages is not over.

But, despite the confused and shifting situation within Iran, administration officials continued to insist that they have reason to believe that Iranian authorities intend to move quickly to gain control over the hostages.

A senior White House official, who declined to be identified, said the United States had "assurances" other than Bani-Sadr's public speech in Tehran yesterday about the sincerity and firmness of the Iranian government's intentions.

However, the official declined to spell out what these assurances were or to predict when the hostage turnover, originally expected early Monday, will happen.

The constantly changing signals coming out of Tehran were described by the official as a reflection of the struggle for power by the different factions there and the maneuvering that the Iranian government must go through to gain the upper hand over the militants.

The official said the administration cannot predict with certainty how the struggle will turn out, and he conceded that this latest attempt to break the 5-month-old impasse could end in the same failure that overwhelmed earlier efforts to resolve the crisis.

But, the official added, the administration believes it now has more grounds for optimism than in the past. And it was optimism that Carter stressed in his public utterances yesterday -- first in an unusual, early-morning televised statement from the White House Oval Office and later in a speech to the AFL-CIO building and construction trades department.

His remarks, coming on the day of the key Democratic presidential primary in Wisconsin, contained a large quotient of campaign rhetoric, including an effort to underscore how he has "worked day and night, literally" to secure the hostages' release.

The White House statement, followed by a brief exchange with reporters, took place at 7:20 a.m. and was in response to the speech made earlier by Bani-Sadr. Carter called the Iranian president's statement" a positive step," and added: "In light of that action, we did not consider it appropriate now to impose additional sanctions."

Last week, Carter sent two messages to Bani-Sadr warning that, unless actions were taken on the hostage issue by March 31, the United States would move to tighten the economic squeeze it has been applying against Iran since early in the crisis.

But, while these new sanctions are being shelved for the moment, Carter also warned: "We will monitor the situation very closely . . . We do not consider it necessary at this time to impose additional sanctions, but that is always an option open to us."

He repeated essentially the same message in his speech to the AFL-CIO officials.But when reporters at the White House session asked whether Carter had agreed to Bani-Sadr's demand that the United States take no hostile action, including propaganda, against Iran, the president dodged the issue by saying only that he would hold off on imposing new sanctions.

Last night, after reports from Tehran said Bani-Sadr had described Carter's responses as failing to meet the Iranian conditions, the senior White House official said Washington had not yet received any official message from Iran to substantiate Bani-Sadr's reported dissatisfaction, and was unclear about what additional assurances, if any, the Iranians want.

The official said the administration was fully aware of the volatile political situation in Iran and wanted to cooperate in strengthening the hand of Bani-Sadr and his moderate allies. But, the official added, there were limits to the restraints Carter can put on his references to the plight of the hostages, and the Iranians are aware of that fact.

The official's comments suggested more flexibility in the American position than had been evident earlier in the day at the White House. In the morning, the same official had said that Carter's essentially evasive response to questions about meeting Bani-Sadr's demands constituted the U.S. position.

But last night, this official stressed the administration's willingness to cooperate with Iranian authorities, and said that the U.S. government was waiting for further clarification of what the Iranians are seeking.

The official also confirmed that during the day U.N. Secretary General Kurt Waldheim had relayed to Secretary of State Cyrus R. Vance the substance of a conversation Waldheim had yesterday with Bani-Sadr.

According to the official, Bani-Sadr told Waldheim that he was still waiting for an American statement in response to his demands, and that Vance had replied, in effect, that "we're not sure of what sort of assurances they are looking for."

A Washington Post report from Tehran last night said that a message underscoring that point was to be delivered to Iranian authorities by the Swiss Embassy in Tehran today. However, the White House official said he did not know whether such a message was in the process of being transmitted.

The official pointed out that the White House had faced a similar, seemingly unsettling setback early yesterday when it received word that Iran's most powerful leader, Ayatollah Ruhollah Khomeini, had made a statement that appeared, at first glance, to oppose the transfer of the hostages.

"That fell heavily on the hearts of those assembled here," the official recalled. "It was probably the longest 45 minutes or one hour of the entire last week or 10 days."

But, he added, after closer inspection it was realized that Khomeini's statement made no mention of the hostages, and U.S. officials concluded that, despite its outwardly negative tone, it was not contradictory of Bani-Sadr's statement that the hostages will be transferred.

Reports from Tehran have suggested that, in effecting the change of custody, Iranian authorities are leaning toward the idea of keeping the hostages inside the U.S. Embassy compound, where they have been confined since Nov. 4.

There also have been suggestions that the Iranian government will permit all or some of the militants to remain in the compound in some sort of joint-custody arrangement.

Asked if that would be acceptable to Carter, the senior official said, "It would have to be a real transfer -- one that clearly puts the hostages under control of the government and not the militants." He added, though, that Carter will wait to see what happens and "then make a judgment about whether it meets our requirements."