The State Department yesterday condemned as "absurd" a move by nonaligned nations to expel Israel from the United Nations, warning that it could have "grave consequences" for the world body if pressed.

The unusually strong statement, by State Department spokesman William Dyess, followed Friday's adoption of an anti-Israel communique by foreign ministers of the 92-nation nonaligned movement, meeting in New Delhi.

It also followed a campaign statement last Oct. 1 by Ronald Reagan pledging that he would suspend U.S. support for the United Nations if Israel were barred. At the time, the GOP presidential nominee criticized President Carter for saying in only general terms that the expulsion of Israel would "raise the gravest questions" about the future of the U.N. General Assembly and U.S. support.

Yesterday, Dyess said it would be "illegal" to challenge Israel's credentials in the General Assembly. He added that the United States would oppose such a challenge "in the firmest and most vigorous way."

If such action is pressed, Dyess continued, it would have "the gravest consequences for United States participation in the General Assembly and for the future of the United Nations itself." He refused to elaborate.

The communique of the nonaligned nations, which was adopted by consensus, urged member states to vote against accepting the credentials of the Israeli delegation at the next meetings of the United Nations or its specialized bodies. Proponents said they took the stand because of what they described as Israel's violation of international law and its annexation of Jerusalem.

A challenge to Israel's participation in the United Nations Educational, Scientific and Cultural Organization (UNESCO) last summer was turned back after worldwide diplomatic efforts by the United States. Israeli credentials were attacked on grounds that they had originated in Jerusalem, thus implying recognition of Israeli annexation. The United States argued successfully that the Jerusalem question would not be prejudiced by the action on the credentials.

Expulsion of Israel from the United Nations would be subject to a U.S. veto in the Security Council. However, opponents of Israeli participation have claimed the right to contest its delegation's credentials in the General Assembly, where nonaligned nations are in the majority and there is no veto.