The United States is holding up a decision on whether to sell iindia 20 tons of uranium for a nuclear power plant until it sees what position the new government there will take on developing nuclear weapons.

The Nuclear Regulatory Commission, which must license all exports of uranium, asked the State Department this week for its assessment of where the week-old government of Prime Minister Haran Singh stands on making nuclear weapons.

"We have to see what the new government's policies are" before any new shipments of uranium can be sent out, said James R. Shea, director of the NRC's Office of International Programs.

While Singh has taken no public stance on the atomic weapons issue, reports from New Delhi indicate that he is moving away from the position against nuclear weapons of former prime minister Morarji Desal and intends to keep his nuclear options open.

American officials here fear that the widespread belief that Pakistan is moving ahead on a program to make atomic weapons will push India - which exploded a nuclear device in 1974 and is technologically able to build atomic weapons now - in the same direction.

"I would not be suprised if India moves to build its own bomb shortly," said one U.S. analyst who keeps a closed eye on nuclear developments in India.

Until now Desai has acted as a brake on an Indian atomic weapons program. He steadfastly resisted on moral grounds the strong pronuclear lobby of scientists and military leaders in India who want to go ahead with a bomb-building program.

State Department and intelligence experts here believe that other Indian political leaders do not have the same commitment against atomic weapons as Desai, whose government fell July 15.

The nuclear issue is the only major divisive element in U.S.-Indian relations.

India has refused to sign the Nuclear Nonproliferation Treaty on the ground that it allows the nuclear powers to go ahead with weapons development while discriminating against those nations that are not members of the atomic club.

India has consistently complained that the treaty discriminates even against the peacful nuclear activities of nations that do not have atomic weapons. New Delhi cites as evidence the delays in shipping nuclear fuel for its power reactors while China received clearance almost immediately to buy reactors from France with no guarantees of international inspections as required by the treaty.

India has a growing and highly sophisticated nuclear power program with eight reactors across the country.

The 20 tons of uranium that is seeks to buy are for one of those reactors, at Tarapur, near Bombay. The United States has contracted to supply all Tarapur's fuel needs until 1993.

A new federal law, however, prohibits the United States from shipping nuclear fuel after next March to any country that refuses to allow international inspections of its nuclear facilities. India has objected to this "safeguard" requirement on the ground that it discriminates against the nonnuclear powers and insults India's national honor.

While balking at safeguards for all its nuclear facilities, India has allowed the inspection of the Tarapur reactor as called for in its contract with the United States. That contract also bars India from using any of the nuclear material for Tarapur or the facility itself in weapons development and from reprocessing the spent nuclear fuel to make plutonium, which can be used for weapons.

If the United States refuses to ship any more uranium for Tarapur, through, there are indications that India will be longer feel bound by those restrictions.

U.S. diplomats, unable to budge India from its stand not to allow safeguards on all its nuclear installations, have concentrated on trying to persuade it to keep the restrictions on Tarapur even if the United States cuts off uranium shipments in March.

Desai said he would not approach any other country for uranium while the contract with the United States was still in force. If the United States stops supplying urnaium for Tarapur, India will either have to get fuel from another country or reprocess the spent fuel.

India takes the position that the United States has a binding contract with India to supply uranium for Tarapur that cannot be amended by any American laws. United States officials say that the new law passed by Congress bars any future shipments of uranium to India unless it agrees to safeguard provisions.