President Carter, confronted by growing threats to the hostages at the U.S. Embassy in Tehran, met unexpectedly with the National Security council last night to review efforts to win their release in the wake of the Iranian government's resignation.

Coincidental with the NSC meeting, reports began leaking out of U.S. government and business circles of interruptions in oil shipments from Iran's main export terminal. However, these reports left unclear whether Iran might be instituting an oil boycott against the United States.

Reliable sources said later that the NSC meeting -- Carter's second of the day with his top security advisers -- was not connected to the oil situation. They described the meeting as part of "the running effort to grapple with the hostage problem" and said it has not produced any changes in the administration's basic strategy.

Specifically, the sources said, the administration is continuing to rule out the idea of military action to free the 60 to 65 American hostages. As a source put it after the meeting, "The upshot was no change in the status quo -- no military alert, no movement of forces, no resort to military contingency plans."

Instead, the source continued, the administration will keep pursuing what they called "every possible and indirect means" of appealing to Iran's unofficial ruler, the Ayatollah Ruhollah Khomeini, to secure the prisoners' safe release.

A spokesman said last night that the State Department is advising U.S. firms with staff in Iran to begin moving them out of the country "in an orderly and quiet fashion." The spokesman said this was being done as a "prudent measure," because the Uniited States is unable to provide normal consular protection for the estimated 300 to 500 U.S. citizens working for American companies in Iran.

Earlier yesterday, the White House issued a statement calling on Khomeini and the other Moslem religious leaders controlling Iran to honor commitments made by the fallen government about the hostages' safety. The statement said: m

"The United States has been given assurances by the authorities in Iran that the safety and well-being of Americans will be protected. The United States expects that these assurances will be honored."

But, U.S. officials conceded, their efforts to break through the political chaos sweeping Iran and make contact with Khomeini and his Islamic Revolutionary Council have been unsuccessful so far.

In fact, they added, the attempt to free the hotages, who were captured Sunday by an Iranian student mob demanding the forced return of the deposed shah to stand trial as a criminal, has been impeded by the resignation yesterday of Prime Minister Mehde Bazargan's weak government.

Until the Revolutionary Council installs a new government, the officials said, the United States can only mark time and try to deal with low-level career Iranian bureaucrats, whose authority is far less than even the limited powers held by the Bazargan government.

Adding an ominous and confusing new note to the situation were revelations that the Central Intelligence Agency told the administration and members of Congress yesterday that a number of Western oil companies had been informed that, beginning today, they could not load oil at Iran's chief oil terminal, Kharg Island.

"The CIA reported this to the Hill," said one State Department official, adding that there is still no information on why the cutoff was imposed or how long it will last.

"The halt in shipping apparently is total. It is not just against the United States," the official said.

State and Energy Department officials said it is unclear whether the halt in oil shipment was ordered by Khomeini or was the result of labor unrest that has gripped the oil fields since the Iranian civil unrest began more than a year ago.

A cutoff in Iranian oil supplies to the United States would not mean an immediate return to gasoline lines like those of last summer, but it would result in another wave of stiff oil price hikes, according to industry analysts.

John Lichtblau, head of the Petroleum Industry Research Foundation in New York, said a shutdown in Iran's oil shipments would not cause gas lines "in the near future."

On Sunday Iran's oil minister, Ali Akhbar Montifar, said he was prepared to ban oil shipments to the United Stattes if ordered to by Khomeini.

Iran now provides the United States 400,000 barrels of oil a day, less than 5 percent of domestic consumption.

At a State Department briefing yesterday, spokesman Hodding Carter told reporters: "We expect that our oil supply will not be cut off."

Privately, though, State and Energy Department officials have been taking a more pessimistic view. Hours before the CIA report was passed around in administration circles, one State Department official said, "The possibilities of a cut off . . . or a drop in (Iranian) exports is very, very great."

State Department officials say that Iran can afford to slash oil exports because it has $9 billion in official reserves, and that Tehran's troubled central government is expected to end the year with a $4 billion surplus.

In the face of the increasingly discouraging Iranian situation, U.S. sources continued to stress privately that the administration's only real option is to try and persuade Khomeini that disputes should be settled not by force, but through negotiation and diplomacy.

In pursuit of that effort, the sources said, Washington is relying on its own direct contacts with Iranian officials and on trying to enlist other governments and individuals as intermediaries.

U.N. Secretary General Kurt Waldheim announced yesterday that he will make a major effort to obtain the hostages' release. Waldheim said through a spokesman he had started talks with Iranian deplomats at the United Nations. semphasizing the need for member states to adhere to international agreements protecting diplomats.

Washington also is understood to be seeking help from such countries as France, which gave Khomeini asylum when he was in political exile, Sweden and various Middle East countries that share Iran's Moslem heritage. The problem, U.S. sources said, is that Iran's relations with many of its neighbors and with other strongly Moslem nations such as Saudi Arabia are strained to the point where these countries have little influence in Tehran.

One group that appeared last night to be leaping into the situation on its own was the Palestine Liberation Organization, whose struggle against Israel has received strong support from Khomeini. PLO spokesmen said the organization's leader, Yasser Arafat, was sending a delegation to Tehran to try to intercede for the hostages.

The announcement said Arafat made the move on his own initiative, without informing the United States. Under U.S. agreements with Israel, the United States does not recognize the PLO, and U.S. officials are prohibited from dealing with the organization.

In the meantime, the United States reiterated yesterday its rejection of Iranian demands for the extradition of Shah Mohammed Reza Pahlavi, who is undergoing treatment for cancer at a New York City hospital. Asked if the shah might be forced to leave, Hodding Carter replied: No! he's going to stay here for medical treatment until it's completed."

In regard to scattered demands that President Carter "send in the Marines," administration sources said the United States is facing a gun-to-the-head situation and that Khomeini's followers in the embassy would almost certainly kill the hostages if threatened by any U.S. military moves.

The sources conceded that this situation does not enhance the image of "resolve" Carter has been trying to project. But, they added, that is not a novel phenomenon in this nuclear age.

They noted that President Johnson could not find a way of using superior U.S. power to retrieve the USS Pueblo when that ship was hijacked and its crew held hostage by North Korea in 1968.