Radical students today rejected a promise made earlier by Iran's foreign minister that some of the 50 American hostages they are holding in the U.S. Embassy here would be freed soon.

"We will release nobody, nobody at all," said a spokesman for the students soon after Foreign Minister Sadegh Ghotbzadeh told a news conference this morning that American diplomats who the Iranians decide are not spies will be released "as soon as possible . . . not very far away from now."

The Foreign Ministry made no comment on the student rejection. The students also said they had made no decision on Ghotbzadeh's pledge that visitors would be able to see all of the hostages.

Once again, this latest conciliatory move appears to have foundered on the wide differences on the hostage issue that are splitting the main ruling faction of this country. Ghotbzadeh is the third foreign minister -- the other two lost their jobs over this issue -- to have tried to get some hostages released only to have the move thwarted by the students.

When 13 black and women hostages were released last month, the students and the Iranian government leaders disagreed. According to sources here, it took the direct intervention of Ayatollah Ruhollah Khomeini to persuade the students to let the hostages go.

It is generally agreed here that only Khomeini can persuade the students to give up the hostages, but until now he has been stridently anti-American and for the most part taken the side of the students.

In a radio broadcast today, Khomeini urged Iranians to "rub America's snout in the dust," and he told them to go to their rooftops tonight to shout praise to Allah and "death to the world-devouring American imperialists." At 9 p.m., the air was filled with shouts.

Yet diplomatic observers here said Ghotbzadeh's statement could be meaningful, especially since he visited Khomeini at the holy city of Qom yesterday and was reported to have seen the students at the U.S. Embassy on Wednesday.

"Be optimistic, but keep some caution in your story," advised one diplomat from a Moslem country. "Much depends on the attitude of the United States to a partial release of the hostages."

Ghotbzadeh was unusually clear and unequivocal in his statement about the possible release of the hostages.

He said those hostages "not consciously involved in espionage, I assure you they will be freed -- no problem." He added that those "who are guilty of espionage and have reached the limits of diplomatic immunity . . . those people are not going to benefit from diplomatic immunity."

Ghotbzedh left it unclear today whether all the hostages would have to face revolutionary court trials before any would be released or whether those judged not to be spies will be let go without a trial.

He said he will make an announcement by Saturday about the trials, which if held, he said, would be before a revolutionary court judge with the students taking no active role in the prosecution.

The question of spies operating under diplomatic cover in the embassy has become a major issue here. The students have claimed that three American diplomats are CIA agents.

The three accused men are Thomas Ahern Jr., who purportedly had a forged Belgian passport and details of his cover identity in his office, and William Daugherty and Malcolm Kalp, who, the students say, were identified in a top secret cable as being CIA agents.

The students released another document today that they said shows that the U.S. Embassy here was "a nest of spies." It is purportedly a memo from Air Force Col. Thomas E. Schaefer, the defense attache here, setting down rules for giving preferential treatment for U.S. visas to Iranian diplomats, and high-ranking military and police officers in return for intelligence information.

"Visa referral can be very valuable to the defense attache's office for getting information not normally accessible through other means," said the alledged memo, which was stamped "Secret" and dated Sept. 18, 1979.

"Contacts, are important, but only if they provide us information or open doors that will lead to valuable intelligence. I expect quid pro quo from these contacts and information that will show up in intelligence reports."

There have been rumors here that eight of the hostages -- presumably cluding Schaefer, Ahern, Daugherty and Kalp -- have been put under extensive interrogation by the students. The students have also said they want to try as spies embassy charge d'affaires Bruce Laingen and two other top embassy officials -- political officer Victor Tomseth and security officer Michael Howland -- who have been held in the Foreign Ministry since the embassy takeover 34 days ago.

The trials themselves appear to be a rollback from the students' original demand that the hostages will not be freed until the United States turns over the ailing and deposed shah to Iran for a trial here.

Observers here believe that government officials and some of the students now realize the United States will not force the shah to return to Iran. Ghotbzadeh in his press conference this morning, for instance, insisted at first that there were no new developments because the United States was "stubborn" in refusing to turn over the shah.

Yet later, he softened that somewhat with language that indicated to diplomats here that there is some maneuvering room to allow Iran and the United States to cut a deal that would free the hostages.

In another conciliatory gesture, Ghotbzadeh said visits to the hostages will be allowed soon, as demanded by the United States and requested by diplomats here. "The principle has been decided," he said. "Visits will occur."

Three weeks ago some diplomats were allowed into the embassy compound and saw some of the American captives. President Carter has accused the students of maltreating the hostages by keeping their hands tied and isolating them from each other.

Ghotbzadeh said the students thought there was no need to allow visits until "everyone slandered us around the world" by charging mistreatment. v"We decided to let the world see for themselves," Ghotbzadeh said. But once again students in the embassy indicated they might not permit visits.

Students talkng to Swedish newsmen said the hostages are allowed 15 minutes of exercise, including some jogging, each day. They get three baths a week and receive a medical checkup once a week from a private Iranian physician. The students said no one is seriously ill and there are no cases of mental disorder.