The United States concluded an agreement with Iran Monday to initiate the process of freeing the 52 American hostages held in Iran for 14 1/2 months.

Deputy Secretary of State Warren Christopher, who headed the American negotiating team, signed the accord at the Algerian Foreign Ministry in Algiers. A small group of reporters witnessed the signing.

Christopher signed three documents, initialing each page, The Associated Press reported from the Algerian capital.

He declined to say exactly what the documents contained, but described the accord as "the beginning of an execution process involving action on three continents."

There was no immediate indication of when the hostages would be flown to freedom. Earlier Monday, two Algerian Boeing 727 jets flew to Tehran after a refueling stop in Ankara, Turkey, and there was wide speculation that the planes will pick up the hostages.

"The hostages are now in Tehran and are being prepared for their journey home," a senior Iranian official told United Press International in London by phone.

Iran's chief hostage negotiator, Behzad Nabavi, scheduled a press conference in Tehran for 6:30 a.m. EST, when he was expected to confirm Iran's signing of the agreement.

In Algiers, preparations continued for receiving the hostages at what the Foreign Ministry spokesman said would probably be their first stop after their leave the Iranian capital, Washington Post correspondent Ronald Koven reported.

The preparations coincided with the conclusion of virtually around-the-clock negotiations between the United States and Iran through Algerian mediators.

U.S. expectations that the end of the hostages' 14 1/2-month ordeal was imminent rose shortly after midnight Tehran time when Nabavi, announced that Tehran and Washington had reached agreement on "all the terms" for release of the 52 Americans. That statement went beyond his remarks Sunday in which he had said Iran and the United States "finally reached agreement on resolving the issue of the hostages." He added at that time that "several wholly trivial points remained" to be resolved by the two governments through their Algerian intermediaries.

Nabavi's subsequent comments, in interviews carried by the official Pars News Agency and Tehran radio, appeared to remove the remaining doubts that an accord had actually been achieved. Tehran is 8 1/2 hours ahead of Washington.

In remarks that allayed the suspicions of some observers that Nabavi was engaging n a last-minute ploy to win U.S. concessions on the unspecified outstanding points, Nabavi said a new American message, apparently received from the Algerians around midnight, had settled those issues.

"I was just informed that the American government has accepted our final views and proposals about the method of implementation and has declared its readiness to sign the Algerian statement," Nabavi told a radio interviewer shortly after midnight, according to Reuter news agency.

"In this way, I think the saga is nearing its end," he said.

Nabavi said in reply to a question that the minor issues had been settled. He added that the Algerians had just told him "that the American government had declared its readiness to sign the statement of the Algerian government, with amendments by us, and has started transferring our assets to the central bank of a third country".

In a separate statement made to Pars at about the same time, Nabavi declared, "The United States had finally accepted all the terms set by the government of the Islamic Republic of Iran for the release of the American hostages." He said the agreement had not yet been signed, but that details of it would be revealed at a news conference later Monday, Reuter reported.

Nabavi did not make clear exactly when the hostages would be freed.

As of 1 a.m. Monday Tehran time (4:30 p.m. Sunday EST), a group of six Algerian doctors who arrived in Iran Saturday to examine the hostages before their release still had not seen the captives, one of the physicians said in a telephone interview. He said that plans to visit the Americans were still being made.

Shortly afterward, CBC News reported that one of the Algerians said the group was now scheduled to see the hostages at noon Monday (3:30 a.. EST). There were unconfirmed reports early Monday that the doctors had left their hotel for an unconfirmed destination.

Asked in the Tehran radio interview whether the hostages would be freed in a few days, Nabavi replied, "Yes, naturally," Reuter reported. "According to the agreement reached, after the statement has been signed and the Algerian government announces that the agreed assets are sent to the central bank of a third country, the hostages will be immediately released."

Nabavi added that the Iranian assets unfrozen by Washington would be transferred "to the central bank of England, but the assets will be at the disposal of the Algerian government."

The remarks were unusually explicit. In the past, Nabavi had confounded reporters with vague pronouncements on the hostage negotiations that left room for widely varying interpretations.

Such was the case only hours before his midnight statements announcing an agreement. In his earlier statement to Pars at 6:30 p.m. Sunday local time (10 a.m. EST), Nabavi initially stated flatly that an accord had been reached, then spoke of "points on which the two sides had reached agreement."

According to Pars, Nabavi added that "there only remain several wholly trivial points which had to be agreed upon and which had been created merely as a result of modifications received in the U.S. proposals today [Sunday]." Pars further quoted Nabavi as saying that "proper replies to the most recent U.S. proposals" had already been sent.

Since Nabavi did not say what the "trivial points" were, his remarks aroused concern that more substantive issues might be involved, entailing further delay in concluding the deal.

The concern was heightened by a Pars dispatch only minutes before in which Nabavi accused Washington of taking "a capricious approach" to the negotiations, with "continuous vacillations which take place always at the 11th hour."

In this more explicit statements early Monday, however, there was no trace of such criticism.

Nabavi, a close aide of Prime Minister Mohammed Ali Rajai, with whom he spent time in prison under the rule of the late shah, has risen to great prominence in Iran since he was put in charge of hostage negotiations last fall. Last summer, when Rajai appointed him to his government, few people in Iran had heard of Nabavi.

Nabavi's great influence became evident when he accompanied Rajai on a visit to the United Nations in New York last October. A mysterious, bearded figure habitually garbed in an olive drab combat jacket, Nabavi frequently whispered advice into Rajai's ear as the Iranian prime minister answered questions at a U.N. news conference.

The key role of Nabavi in the recent hostage dealings eclipsed that played earlier by Iranian officials regarded as more "moderate" than the men around the fundamentalist Rajai.

President Abol Hassan Bani-Sadr, formerly a principal advocate of settling the U.S.-Iranian dispute, has been dealing almost exclusively with the war with neighboring Iraq since it erupted Sept. 22.

According to the Pars News Agency, Bani-Sadr conferred Sunday morning with Iranian revolutionary leader Ayatollah Ruhollah Khomeini. No details of the meeting were disclosed, but it was believed to concern developments in the war.

Neither Khomeini nor Bani-Sadr immediately mentioned the U.S.-Iranian agreement publicly, and the government at first appeared to be giving it short shrift on the state-run news media. Until its interview with Nabavi broadcast after midnight, Tehran radio Sunday had not mentioned any hostage agreement.

Mondy, Tehran's Kayhan newspaper headlined its report on the announcement that agreement was near, "It's Over! The Great Satan Bows to All our Conditions."

There was no immediate public reaction in Tehran to the prospect of imminent freedom for the hostages.

Earlier in the day, the Iranian parliament, the Majlis, had held a regularly scheduled session in which the issue never came up. In any case, opposition to a settlement by hard-line Majlis members appeared to die out after the assembly approved four conditions on Nov. 2 for the Americans' release.

Furthermore, by Christmas it appeared evident that the hostages had been transferred from the control of the militant Moslem students who seized them Nov. 4, 1979. In a film that seemed to bear out the militants' claims to have turned the Americans over to government control shortly before, the hostages were shown speaking more freely than they had in the past, and some of them appeared in surroundings identified as a resort hotel on the Caspian Sea coast in northern Iran. In addition, there was a noticeable absence of the militants' usual propaganda props such as posters and slogans in the background.

Since then, moreover, there have been unconfirmed reports from Tehran that some of the militants have been killed in the war with Iraq.