MONTREAL (AP) _ Iran failed Thursday to get a clear condemnation of the United States for the July 3 attack on an Iranian jetliner, but a special session of the International Civil Aviation Organization said it deplored the use of weapons against a civil aircraft.

The United Nations agency's 33-member governing Council unanimously agreed to an independent ICAO investigation into the disaster, with results to be reported at the next ICAO session Sept. 15-16.

''We are delighted by the results,'' U.S. Assistant Secretary of State Richard S. Williamson said at the end of two-day meeting. ''Temptations and efforts to politicize this session were rebuffed.''

Iranian spokesman Seirous Naseri said his country had hoped for a strong condemnation of the United States and warned that the July 3 tragedy could recur unless U.S. forces withdraw from the Persian Gulf.

Naseri blamed ''political constraints'' for the Council's mild response, citing American ''influence and power.''

''It is not easy for all countries to come out and speak very frankly and openly against the United States, even when the United States commits a very serious crime.''

He said Iran would look to the U.N. Security Council for political condemnation of the U.S. action.

Williamson rankled when asked at a news conference if he had engaged in arm-twisting. ''That's a ridiculous and insulting assertion by the Iranians,'' he said. Naseri told the Council that on July 5, two days after the U.S. warship Vincennes shot down Iran Air flight 655 killing all 290 people on board, a domestic Iranian flight was warned by U.S. forces in the gulf.

''Fortunately this time the warning was received by the pilot and he was able to respond,'' Naseri said. ''This could have ended in a similar tragedy.''

The Iranians maintain that Iran Air 655 never received the seven military and civilian channel radio warnings the Vincennes claims it sent before firing.

Naseri said he was not aware of why the pilot of the later flight, Iran Air 235 from Bandar Langah to Bandar Abbas, heard the U.S. warning when the doomed earlier flight apparently did not.

Williamson said he was not aware of the second incident.

The ICAO decision was in the form of a nine-part statement from Council President Dr. Assad Kotaite.

The Soviet Union, Czechoslovakia, Cuba, China and Kenya wanted to back Iran more strongly by calling the statement a ''resolution'' but they were outvoted.

The decision read very similarly to ICAO's response in 1983 after the Soviet Union shot down a Korean Air Lines jet, and that was called a resolution. Following an investigation, ICAO did condemn that Soviet action.

In Thursday's decision, members asked the ICAO secretary general to launch an immediate fact-finding investigation to determine ''all the relevant facts and technical aspects of the chain of events relevant to the flight and destruction of the aircraft.''

All relevant parties were urged to cooperate fully with the probe. Iran and the United States said during the session that they would.

''The Council deplored the use of weapons against a civil aircraft,'' the decision said. It expressed ''profound regret'' at the loss of lives and deep sympathy with the victims' families.

The Council reaffirmed the principle that ''states must refrain from resorting to the use of violence against civil aircraft,'' appealed to more nations to ratify a 1984 ICAO protocol to that effect, and asked countries to refrain from jeopardizing the safety of civil aviation in troubled areas.

The decision also called on the ICAO to renew efforts to coordinate civilian and military air traffic, improve routing in troubled areas and review standards and practises.

Naseri said rerouting Iranian flights could lead to chaos. ''It would be the decisions of military commanders somewhere in the world to place their ships wherever they want and then decide routes would be adjusted accordingly,'' said Naseri, Iran's U.N. representative in Geneva.