The Reagan administration, which is reassessing U.S. participation in the International Atomic Energy Agency, has decided to suspend paying the remaining $8.5 million due on its 1982 agency assessment, sources said yesterday.

The U.N. specialized agency three weeks ago rejected the credentials of Israel, prompting the United States to walk out in protest.

The administration, in making the new decision to cut off the atomic agency's funds, appears to hope this tangible demonstration of U.S. anger over the politicization of the agency might have an impact in Nairobi, where a vote is expected early next week on an Arab attempt to expel Israel from the International Telecommunications Union.

The latest cable from the U.S. delegation attending the Nairobi conference expressed concern that Israel will be expelled by a three- or four-vote margin, sources said yesterday. The cable urged a public announcement by the United States of a cutoff of funding to the atomic energy agency to demonstrate that the Reagan administration is not going to tolerate the politicization of world organizations.

But by last night, no U.S. announcement of the decision to suspend payment of funds to the atomic energy agency had been forthcoming. There was no immediate explanation of why the action had not been announced, or when an official statement might be expected.

State Department sources said, however, that a decision has been made that if Israel is expelled from the International Telecommunications Union, the U.S. delegation will be pulled out of that conference just as the American delegates walked out in support of Israel in Vienna.

A walkout in Nairobi might also have further-reaching implications since the crucial vote there apparently will come on an Algerian resolution to expel Israel. The critical vote in Vienna was to reject Israel's credentials for the balance of the conference, leaving its membership in the atomic energy agency intact.

Thus, the United States might find it more difficult to continue participation in the International Telecommunications Union, particularly in light of resolutions overwhelmingly passed by Congress earlier this year urging that the United States remain out of any U.N. bodies as long as Israel was barred from participation.

The International Telecommunications Union, like the atomic agency, is a U.N. specialized body.

In addition to freezing the final assessment payment to the atomic energy agency, sources said, the Reagan administration has also decided to suspend payment of the final $300,000 of a $4.2 million voluntary contribution it had agreed to make in 1982 to the agency's technical assistance fund.

Both sums, sources said, will be withheld at least until mid-November, when the Reagan administration's reassessment of the U.S. relationship with the atomic energy agency is scheduled to be completed. Diplomatic sources noted that by that time, an effort expected late this month to reject the credentials of Israel at the U.N. General Assembly will already have occurred.

The U.S. ambassador to the United Nations, Jeane J. Kirkpatrick, has pressed for a hard line in dealing with the atomic energy agency, arguing that the moves against Israel in that forum and at Nairobi are precursors of broader Arab efforts.

A number of administration and congressional officials, however, are worried that the Reagan administration's actions have the potential of seriously undermining the only world body charged with attempting to prevent nuclear proliferation.

The United States not only provides 25.8 percent of the funding for the atomic energy agency but also plays a crucial role in training the inspectors and developing the technical equipment used in attempting to ensure that countries are not diverting material from civilian nuclear reactors for use in attempting to acquire atomic weapons.

Hans Blix, director general of the atomic energy agency, sent a message to the United States yesterday expressing concern over punitive measures against the agency, and warning that they would have a "serious impact" on its operations, according to diplomatic sources.

While the agency has not yet felt the financial impact of the U.S. decision to withhold part of its assessment, which sources said would normally have been made toward the end of October, the United States for the past three weeks has been boycotting agency meetings and activities. It also has suspended all visits by atomic energy agency personnel to the United States except for inspectors.