The United States has been holding up the supply of spare parts for India's American-built Tarapur atomic power plant, adding a new twist to a long-running controversy and sparking charges here that the U.S. restrictions are raising the risk of radiation contamination at the plant.

The dispute, U.S. and Indian government sources said, is certain to dominate the agenda of talks here late next month between Secretary of State George P. Shultz and Prime Minister Indira Gandhi.

Indian officials have complained that the United States is reneging on a promise they said was made last year--when responsibility for providing fuel for the plant was transferred to France--that U.S. firms would begin shipping spare parts to Tarapur six to eight weeks after the fuel supply agreement was signed.

The substitution of France as the supplier of nuclear fuel to Tarapur was the centerpiece agreement of talks in Washington last July between Gandhi and President Reagan. The accord signaled a thawing of U.S.-Indian relations after several years of contention over differing interpretations of Washington's obligation to keep the Tarapur facility functioning.

U.S. officials said today that while they are concerned about the safety of the Tarapur reactor--Bombay's primary source of electricity--supply of spare parts can only be made in conformity with the 1978 U.S. Nuclear Nonproliferation Act that bars the resupply of fuel on the basis of India's refusal to accept full-scale international inspection of all its nuclear facilities.

"At no time during the settlement discussions or thereafter did U.S. government officials ever give assurances to the Indian government or otherwise indicate that a supply of spare parts would be made available at any specific time. Moreover, the United States does not have a contractual obligation to supply spare parts for the Tarapur reactor," William Miller, press attache of the U.S. Embassy here, said in response to questions.

Miller said the U.S. government was reviewing whether the export of the spare parts, which include replacement water pumps and components of monitoring devices, can be made in accordance with the nonproliferation act and the Atomic Energy Act.

In Washington, a State Department official said the nonproliferation act covered all materials related to nuclear reactors, including spare parts.

The official said one possible solution to the dispute was a presidential waiver, subject to congressional approval, as was used by the Carter administration for one fuel shipment to India in 1980. But no decision on a waiver has been made, the official said.

Miller, the U.S. Embassy spokesman, denied suggestions that the sale of spare parts was being held up in an attempt to extract from the Indian government promises not to reprocess spent fuel at the Tarapur facility or to force it to submit to other U.S. conditions of nonproliferation.

Under the U.S.-Indo-French fuel supply agreement, Miller said, nuclear material produced at Tarapur may be reprocessed in Indian facilities only with the concurrence of the United States.

Miller acknowledged that India maintains a "different legal view" of the reprocessing provision, and said that discussions on that issue are continuing.

Officials of the Indian Department of Atomic Energy were reluctant to publicly allege a link between the parts holdup and the reprocessing dispute.

But G.K. Reddy, a political commentator for the Hindu daily newspaper who frequently reflects government thinking on major issues, asserted that "the attempt to relink the spare parts issue with the processing controversy, even after separating the fuel supply issue from it, is seen as an obvious attempt to exert fresh pressures on India to submit to the U.S. concepts of nonproliferation."

Responding to persistent reports in the Indian press that employes at Tarapur have been exposed to dangerous levels of radiation as a result of leakages caused by a lack of spare parts, H. N. Sethna, chairman of the Department of Atomic Energy, said in a press conference in Bombay yesterday that a lack of parts was "a matter of concern" but the safety of employes was not in danger.

Sethna said some employes had been exposed to radiation above the acceptable 5,000 millirems a year, but he maintained that the effects of radiation over a long period of time had not been adequately studied, and that an individual could be safely exposed to 10,000 millirems at one time or up to 100,000 millirems over a lifetime.

A millirem is a unit of effective radiation absorbed by body tissue.

U.S. and Indian officials said that India's concern about the spare parts holdup had been expressed in detail to U.S. Ambassador Harry G. Barnes Jr. before he left for Washington last month to prepare for Shultz' visit.

The secretary of state is due here after attending the Association of Southeast Asian Nations conference June 27-28 in Bangkok.

He is also scheduled to visit Pakistan.