In an effort to limit reporting on the Americans being held as prisoners by Islamic extremists inside the U.S. Embassy in Tehran, the Iranian government has expelled all American reporters, including correspondents of The Washington Post. Using reports from those news organizations that are permitted to continue to function in Iran and from information available outside that country, The Post will continue to report the siege of the embassy, which today entered its 88th day.

Six American diplomats who evaded the capture of the U.S. Embassy in Tehran hid in the Canadian Embassy there for three months and then escaped from Iran last weekend by using Canadian passports, U.S. and Canadian officials said yesterday.

The Americans -- four men and two women -- left with Canadian Ambassador Kenneth Taylor and the three members of his embassy staff at a time apparently chosen because the preoccupation of Iranians with their presidential election made the departure less liable to close scrutiny.

Reliable sources said the passports used by the Americans had Iranian visas forged in theem to cover the real identities of the users.

The State Department Identified the six as Mark J. Lijek, an embassy consular official, and his wife, Cora Amburn Lijek, who was employed at the embassy as a consular assistant, of Falls Church; Joseph D. Stafford, a consular official, and his wife, Kathleen F. Stafford, also a consular assistant, of Crossville, Tenn.; Robert G. Anders, another consular official, of Port Charlotte, Fla.; and Henry Lee Schatz, an agricultural attache, of Coeur D'Alene, Idaho.

State Department spokesman Hodding Carter said the six had managed to get away when the mobs of Iranian militants stormed the U.S. Embassy compound Nov. 4 and took the Americans assigned there captive to their demand that the United States return the deposed shah, Mohammad Reza Pahlavi, to Iran as a criminal.

Carter and these six were not among the 50 persons that the department estimates are still held hostage within the compound.

The spokesman added that he could not say what effect revelation of their escape might have on those still in the hands of Iranian militants. But he added that any retaliation against the remaining hostages by their captors "would be an irrational act."

The story of how the six were kept hidden from anti-American Iranian mobs for 12 weeks began to emerge yesterday in the wake of a Canadian government announcement that it had closed its embassy in Tehran.

Initially, the Canadians said that action had been taken as a gesture of protest about Iran's refusal to free the American hostages. However, the Montreal French-language newspaper, La Presse, in a dispatch from its Washington correspondent, Jean Pelletier, quickly revealed that the closing of the Canadian Embassy had been done to cover the Americans' escape.

In the wake of the La Presse article, Canadian External Affairs Minister Flora MacDonald, at an Ottawa press conference, and a number of other American and Canadian officials began, publicly and privately, filling in the details.

Although there were some discrepancies in their various accounts, the information they provided yesterday formed this composite account of what happened:

When the militants attacked the U.S. Embassy on Nov. 4, the six managed, through various means, to avoidbeing captured. Although MacDonald said they were outside the compound at the time of its seizure, Kim King, an American tourist who was present at the time, said yesterday that some of those listed among the escapees were with him in a second-floor visa section within the embassy and that they all fled through a rear door.

In an interview yesterday with Washigton radio station WMAL, Mark Lijek's mother, Wanda Jijek, who lives in Huntington Beach, Calif., said she had talked with her son and he had confirmed that he and his wife were in the embassy when the siege began.

"They were lucky," Wanda Lijek told WMAL. "They were there two hours, and then they walked out a back door and down the street. They ran into some students or revolutionary guards or whatever they're called, but they [the Iranians] just looked at their papers and let them walk away."

Initially, the six took refuge in a variety of friendly embassies, which U.S. officials refused to identify on security grounds. Within a short time, through, they were able to make their way secretly to the Canadian embassy, which hid them in various Tehran residences under its control.

From then on, the presence of the six Americans remained a secret that high-ranking U.S. and Canadian officials sought to keep to themselves while they pondered what to do next. Word about the Americans did leak to some members of Congress and to some journalists, including Pelletier, in the United States and Iran, but they kept the secret.

Canadian Prime Minister Joe Clark, whose Conservative Party is campaigning for reelection, said yesterday that, one week after the Americans arrived in the Canadian Embassy, he told Pierre Trudeau, leader of the opposition Liberal Party, about the situation.

In an indirect confirmation of how successfully the secret was kept, Iranian Foreign Minister Sadegh Ghotbzadeh, when informed of the escape, said yesterday: "I don't know anything about this." His ministry later issued a brief statement saying it was asking Canada for an explanation.

In Tehran, militants holding the American hostages at the U.S. Embassy today demanded to know how the six Americans could slip out of Iran without the government's knowledge, Reuter news service reported.

A militant spokesman told Reuter: "The Foreign Ministry must know the number of diplomats in Iran. So how come more than the known number of Canadian diplomats were able to leave?"

The spokesman said the escape would not affect the treatment of the hostages and added that the militants expect no reprisals against Canadians still in Iran, Reuter reported.

[There was no immediate official reaction in Tehran to the escape and the news was not broadcast on state radio or television, Reuter reported,]

Over the ensuing weeks, U.S. and Canadian officials weighed and discarded a number of strategies for getting the six out of Iran.For a time, some sources said, thought was given to trying to smuggle them across the border into neighboring Turkey, but that idea was rejected as too risky.

Instead, the sources confirmed, the two governments decided to wait for a propitious moment when it might be possible to get them out under the guise of being Canadian diplomats.

To prepare the ground, the Canadians began reducing the size of their embassy staff in Tehran, citing as reasons displeasure over the hostage situation and an inability to conduct normal business in the midst of Iran's political turmoil. However, some of the few Canadian diplomats remaining also began traveling in and out of the country to establish a pattern of movement that would become familiar to Iranian officials.

Although MacDonald and other officials and the two countries refused to talk publicly about details of how the escape was accomplished, reliable sources said the Canadians turned over six passports to U.S. intelligence officials so they could be doctored with false visas and made to appear the property of regular Canadian diplomats.

According to the sources, the passports then were returned to Canada and brought to Tehran in the Canadian Embassy's diplomatic pouch.

Finally, it was decided that the intense excitement that swept Iran last week during the culmination of its presidential election would provide the best moment for the escape attempt.

"We knew that every day the danger was becoming greater," MacDonald said at her press conference yesterday. "Then it was brought to our attention that the country was preoccupied by its own internal election, and therefore we were able to move."

Over the weekend, while the Iranians were celebrating the election returns that gave an overwhelming majority to Finance Minister Abol Hassan Bani-Sadr's bid for the presidency, the Canadians quietly began abandoning their embassy in Tehran.

MacDonald said the six Americans left first, successfully slipping out of Tehran with their Canadian pasports, and were followed shortly afterward by Ambassador Taylor and the three remaining members of his staff.

Taylor is now in Copenhagen.

She added that the Canadaian government felt it had to pull all of its diplomats out because of fears they might be subject to Iranian retaliation. On Monday, Canada announced formally that it had shut down its embassy temporarily, but the announcement made no mention of the six Americans.

Although that action touched off a lot of questions in the Canadian Parliament and press about the reasons for the closure, reliable sources said the Clark government's original intention was to try to keep the escape a secret.

However, that idea had to be abandoned when Pelletier, the son of a Canadiana diplomat, published his article yesterday in La Presse. He said later he had known about the Americans since Dec. 10 but had kept the secret until he knew they were safely out of Iran.

Clark, whose government has been a strong supporter of the United States in the Iranian crisis, said yesterday that he never had any doubt about the correctness of Canada's action, and added that he would have helped any country in the same position.

At the State Department, spokesman Carter expressed the "deep appreciation" of the United States for Canada's assistance, and officials at the Canadian Embassy here said they were flooded with calls of thanks from Americans all yesterday.

State Department officials at first declined to provide any information about the escapees, saying they were trying to protect the Americans' privacy. aBut later they released the names and home towns.

The wife of one diplomat still being held hostage in Tehran said she thought Mark Lijek and Joseph Stafford were relatively young foreign service officers whose wives had gone to Tehran last fall after the ban on dependents was lifted to allow adult dependents to come to the area. Both wives are believed to have taken the consular training course last summer in Washington at the short-handed embassy.

A woman in Arlington said she had sublet her apartment to the Staffords last summer but said she hardly knew them.

The Agriculture Department said Schatz, 31, has been in the department's foreign service since 1975. He arrived in Tehran last August after serving in New Delhi, India. The agricultural office in Tehran is on a side street in the embassy area, so Schatz and some non-Americans in the office apparently were able to escape during the initial commotion at the embassy.