The Iranian parliament failed to hold its scheduled public debate today on terms for the release of the U.S. hostages because not enough legislators turned up to make a quorum, Iran's official radio announced.

The radio said the parliament would hold its next session on Sunday at 8 a.m. Tehran time (11:30 p.m. Saturday EST).

It was not immediately clear whether the failure to attain a quorum was due to a boycott by parliamentarians opposed to the hostages' release or was caused by other factors. According to an Iranian source reached by telephone in Tehran, the parliament has canceled several sessions in the past because too many legislators were away for a variety of reasons. Since the Iranian-Iraqi war began last month, some sessions have been called off because a number of deputies were visiting the front.

Iran's parliament, or Majlis, requires a quorum of 179 before it can go into session. Of the body's 270 seats, 228 are currently filled.

At the end of yesterday's session, parliamentary leaders issued a routine call for deputies to appear for today's debate. The radio today did not immediately announce how many legislators had actually shown up for the debate on a cold, cloudy day in the Iranian capital. One possible reason for the failure to reach a quorum was that Thursday, the day before the Moslem Sabbath, is Iran's equivalent of a Saturday.

The parliament, overriding objections from Islamic conservatives, had decided yesterday to hold a public debate today on the fate of the 52 American hostages amid increasing signs that an assembly majority is inching toward agreement on final conditions for their release.

The decision to open the proceedings, after three days of inconclusive secret deliberations, was interpreted as an indication that a vote might be near on terms for freeing the captives after 361 days in the hands of Iran's Islamic revolutionaries.

After today's cancellation, however, it still remained unclear exactly when the vote will be held -- and even less clear when and how Iran and the United States would agree on steps leading to freedom for the hostages once the vote is completed.

Sources in the parliament told Reuter in Tehran that a majority of the 228-member assembly now seems ready to reject suggestions that the Majlis impose new conditions for the hostages' release in addition to the four laid down by Iran's spiritual and temporal leader, Ayatollah Ruhollah Khomeini.

Khomeini, the 80-year-old father of the Iranian revolution, has said the hostages can be set free if the United States returns to Iran the wealth of the late shah, Mohammad Reza Pahlavi, pledges noninterference in Iranian affairs, withdraws legal action against Iran and unfreezes Iranian assets held in U.S. financial institutions.

But Sadegh Khalkhali, Iran's self-appointed roving Islamic judge and a Majlis member, predicted to reporters in Tehran that the assembly will also require U.S. spare military parts as part of any deal for releasing the hostages. President Carter, in Tuesday night's campaign debate, reiterated that if they are freed he will ship to Iran $400 million in military equipment paid for by Iran but held in the United States because of the hostages.

Although Khalkhali has limited influence in the assembly or the government, his comment marks the first explicit mention from an Iranian official of a desire in Tehran to trade military help in the war against Iraq for release of the hostages.

In the war, now more than five weeks old, the Iranian military command reported shooting down two of Iraq's Soviet-built Tupolev bombers deep inside Iran in one of the rare reports of penetration raids by Iraqi warplanes.

One of the Tupolevs went down near the holy city of Qom, a Shiite theological center 90 miles south of Tehran and Khomeini's headquarters until he moved to the capital early this year for heart treatment, according to a communique broadcast by Tehran radio. The other was downed near Najafabad about 25 miles west of the ancient Persian capital of Isfahan in central Iran, it added.

"All six crew members of the bombers were burned in the fire of Allah's punishment," the communique said.

Air attacks also were reported by both sides in support of ground fighting around the Iranian oil refinery complex at Abadan on the Shatt-al-Arab waterway, where Iraqi forces continued to press their siege against stubborn Iranian resistance.

In an indication that the war may be going badly for Iran, an Iranian radio station lashed out at deserters leaving the trenches at the besieged oil center of Ahwaz, calling them chicken-hearted cowards who sought to hide "like jackals," United Press International reported. The Ahwaz radio station said that a "group of Army personnel" was "resorting to every trick to avoid confrontation with the enemy."

Some hard-line members of the Iranian Majlis had suggested in the first two days of the hostage debate Sunday and Monday that Iran also demand withdrawal of American reconnaissance planes from the Persian Gulf area and a formal U.S. apology for abuses committed during the shah's reign.

The Carter administration has in the past rejected both suggestions. After yesterday's reportedly stormy debate in Tehran, parliament sources said the emerging majority also appears opposed to going beyond Khomeini's list of conditions despite Khalkhali's remarks. Observers in the Iranian capital said the scheduling of a public debate Thursday indicated this majority felt it would get its way.

Most Majlis members remained opposed to the presence of the U.S. radar planes in the gulf area -- where Iranian officials claim they are relaying information to Iraq -- but conceded that demanding their withdrawal would prolong the hostage crisis without additional advantages for Iran, deputies told Reuter in Tehran.

Other parliamentary sources cautioned, however, that with Friday being the Moslem holy day and with pockets of hard-line opposition remaining, the vote could be put off. Washington Post correspondent Ronald Koven reported that several Majlis members interviewed by journalists calling from Paris said they were in no hurry to end the debate despite the impending U.S. presidential election.

The determination of Islamic hardliners, even though they seem outnumbered in the Majlis, was dramatized during yesterday's four-hour debate when a number of them walked out to protest the decision to open Thursday's discussions. Some members are known to favor dragging out the debate in private. One group tried on the first day of procedings to cancel it altogether until after the gulf war is settled.

Khomeini, who has assigned the final decision on the hostages to the Majlis, failed to mention the hostages at all in a speech marking Tuesday's Shiite Moslem holiday.

This was taken as a sign that he may be awaiting a more distinct consensus among Majlis members or that he genuinely intends to leave the decision up the assembly.