Carter administration officials expressed guarded optimism yesterday that growing international pressures, including existing and planned U.S. initiatives, will bring a change of heart in Iran leading to the release of the American hostages.

The officials, including presidential assistant Zbigniew Brzezinski, were careful not to make flat predictions that their strategy for dealing with Iran will succeed. But the tone and substance of their remarks were notably less gloomy as the 33-day-old crisis entered a stage of intensified diplomatic efforts through both open and confidential channels.

"We feel Iran is not immune to the attitudes and actions of the international community," Brzezinski told reporters at a breakfast meeting. While acknowledging that the siutation there is unclear and has been changing rapidly, the national security adviser said that "within Iran there are many sober individuals who must be asking themselves, 'Where is this course of action taking us?'"

Brzezinski added that "more and more Iranians must be concluding that this whole misadventure is costly now and will be very costly in the future." He emphasized the word very -- a hint of growing pressures to come.

White House press secretary Jody Powell denied that President Carter had declardd that U.S. strategy is to "turn the screws a little tighter" on Iran day by day, as he was widely quoted as telling a group of House members Wednesday night.

But neither Powell nor other high officials denied that the application of growing pressures on Iran is an essential part of the U.S. response to the crisis.

The relatively moderate response by Iranian Foreign Minister Sadegh Ghotbzadeh to the United Nations Security Council, which called unanimously for release of the hostages, was termed "interesting" yesterday by the White House spokesman. Powell said there are "straws in the wind" that may indicate new directions in Iran, but that it is too early to tell how important they are.

Secretary of State Cyrus R. Vance's whirlwind trip to European capitals, which was announced yesterday and will take place early next week, is expected to solicit continued allied support for the economic as well as the diplomatic aspects of the administration's effort.

Shipments of U.S. food and pharmaceuticals have dried up due to the freeze on Iranian assets in this country and the refusal by longshoremen to load goods for Iran, according to government reports. Shipments from other supplier countries are also reported to have been sharply affected because of doubts about Iran's ability to pay.

Vice President Mondale, campaigning in Illinois yesterday, said the United States has ruled out using food or medicine as a weapon against Iran. "Our fight is not with the poor people of Iran" who need these items, he said.

A decision by several European countries to embargo the shipment of military goods to Iran is reported to be spreading on the continent, and Vance is likely to encourage this. A number of other measures are under discussion in administration councils, including a possible international ban on landings by Iranian aircraft and possible declaration of a formal economic embargo on Iran until the hostages are freed.

A major part of the diplomatic effort is centered at the United Nations, where Secretary General Kurt Waldheim has been assigned by the Security Council to take an active role. The United States is reported also to have sent diplomatic messages direct to the Iranian authorities through secret channels that were employed near the beginning of the crisis and have been reactivated.

American officials continue to be uncertain about who wields what powers in the highly complex Iranian political situation, which has been made more conplicated by the outbreak of internal troubles between rival ayatollahs. m

Describing the political situation as "extraordinarily abnormal," Brzezinski quickly ticked off these major elements: " a charismatic leader who is not in the cabinet and who is surrounded by a group of confidants, the identity of whom is not in all cases known; the Revolutionary Council, which seems to be fairly fluid; a piece of a government, but without an established prime iminster; ministers who change very frequently; disorganized armed forces; several private armies, and on the fringes, ethnic separatist movements."

Asked about the identity of the militants holding the hostages, Brzezinski said the current U.S. judgement is that the takeover was started by a group of university students who are "fanatically committed, religiously motivated" followers of Ayatollah Ruhollah Khomeini. These have been augmented by "an increasing presence of radical left elements," he said.

According to some reports in the hands of American officials, several different groups of militants inside the embassy compound have their own turfs, with a coordinating committee to make important decisions.

One of the many imponderables being debated in administrations councils is the extent of control that Khomeini exercises or can exercise over the militants at the embassy.Brzezinski expressed a "guess" that Khomeini would be able to impose his will, if a fundamental decision of his were opposed by the "controlling group" in the embassy, because "his influence is still overriding."

Brzezinski said there is "no safe way to predict" what effect departure from the United States of the deposed shah, Mohammad Reza Pahlavi, would have on the hostages, in view of conflicting signals from Tehran.

The shah remained at heavily guarded Lackland Air Force Base in San Antonio, Tex., where he was reported to be reading, playing chess, talking on the telephone, and taking walks with his wife and two pet dogs. He is staying at the visiting-officers' quarters, which have been set aside for him.

A new controversy developed from comments by State Department spokesman Hodding Carter late Wednesday criticizing American television network interviews with Khomeini on Nov. 18.

Carter was quoted by Reuter as telling Princeton University students in a seminar that network interviewers had done "a disservice" to this nation by forcing Khomeini into "a much more rigid position" on the question of placing hostages on trial.

Carter said yesterday that his comments about the networks were in an "off-the-record" seminar where he was speaking as a Princeton alumnus and private citizen rather than as a goverment official. Officially, he said, he has "no assessment" of the networks' performances.