The United States walked out of the General Assembly for the first time in nearly 15 years today when the Iranian delegate took the floor. U.S. Ambassador William Vanden Heuvel said, "We owed it to the hostages."

Vanden Heuvel, deputy U.S. Mission chief, led the walkout when Ali Shams Ardekani, Iranian ambassador to Kuwait, strode to the podium to denounce the United States as an "enemy of peace on earth" during the General Assembly's special emergency debate on Palestine.

Later, coming back to the General Assembly from the U.S. Mission across the street after Ardekani had finished speaking, Vanden Heuvel told reporters he had decided to walk out before the Iranian spoke -- and not because of the content of his speech but because of the 52 Americans being held hostage in Iran for 265 days.

"It was deliberate, I told my senior colleagues in Washington that I was going to do it," Vanden Heuvel said. "I left before he began speaking and came back after he finished. I had no idea what he said . . . I was leaving out of respect for our hostages and to remind all members states here of the obligations all of us have to honor international law."

A press spokesman at the United States Mission said today's walkout was not a boycott. "Other member states, when they walked out, have taken everybody, leaving their seats unrepresented," the spokesman said. "we left a junior officer behind to take notes. The United States was still there and still represented. We were listening to the speaker.

"However, the ambassador felt it was necessary to make some kind of gesture for the 52 hostages in Tehran."

Another mission spokesman said the last time the United States walked out on a General Assembly speaker was on Oct. 15, 1965, when then-ambassador Arthur J. Goldberg turned his back on the Cuban foreign minister and walked from the hall.

The spokesman said that, as far as he knew that was the only other time the United States has walked out of an Assembly session.

Ardekani went on with his speech, calling Israel a "parasite" and saying the United States was on the top of the list . . . of the enemies of peace on earth."

"We need a strong resolution combined with a plan of action to put economic and political pressure on those who helped the Zionist parasitic entity," Ardekani said, referring to both Israel and the United States.

The American walkout was the highlight of the four-day-old debate that is expected to wind up next week with passage of a resolution demanding Israeli withdrawal from occupied Arab territories.

The United States took its turn at the podium on Thursday when, joined by Western Europe, it insisted that the assembly recognize and respect the "reality" of peace.

The nonaligned states' draft resolution being debated at the special session would demand that Israel withdraw from occupied Arab territories, and the United Nations had those territories over to the Palestine Liberation Organization for establishment of an independent state.

Even if such a resolution is adopted, however, the General Assembly has no power to enforce it, and Israel is expected to ignore it.

Clovis Maksoud, Arab League U.N. observer, told the assembly that if Israel kept defying U.N. decisions, "the international community will have no other choice but to withdraw Israel's membership" from the United Nations.

Under the U.N. Chapter, a member that has persistently violated charter principles may be expelled by the General Assembly on recommendation of the Security Council.

The United States could be expected to veto any such recommendation. But according to a precedent set by a 1974 case involving South Africa, a U.N. member can be expelled from any General Assembly session by majority vote if its credentials are rejected.