The United States will buy heavy water from Iran's nuclear program and expects it to be delivered within weeks, US officials said on Friday, a move that Republican lawmakers quickly criticised.

The US Department of Energy (DOE), will buy 32 metric tonnes of heavy water from Iran worth $8.6 million, a department spokeswoman said. Heavy water is a component of making nuclear weapons and nuclear energy, that is not radioactive.

Under last year's landmark nuclear deal between Iran, the US and five other world powers, Tehran is responsible for reducing its stock of heavy water, which it can sell, dilute or dispose of, under conditions.

Iran is permitted to keep up to 130 tonnes of heavy water at present and up to 90 tonnes once its redesigned and rebuilt Arak nuclear research reactor is commissioned.

"The US will not be Iran's customer forever," the DOE spokeswoman said. US officials hope the purchase will pave the way for other countries to buy the heavy water, which can be used in the development of semiconductors and nuclear magnetic resonance imaging.

Iran's top nuclear negotiator, Abbas Araqchi, who discussed the sale with US officials in Vienna on Friday, told reporters that the 32 tonnes have been sold to an American company.

Araqchi estimated Iran has about 70 tonnes in excess of what it needs and said further sales are being negotiated with another company that is not based in the United States.

Iran, which is still under US sanctions, has long had to go through third-country financial institutions for authorized transactions for items including medicine and food.

A US Treasury Department official would not discuss details of the payment for the heavy water until after the purchase is complete, but said it would be completed under the same method.

"Regardless of whether or not this is in US dollars, this licensed transaction is limited in scope," the Treasury official said on condition of anonymity.

Iran's compliance with the nuclear deal means that the heavy water was already removed from Iran, ensuring that it would not be used to support the development of a nuclear weapon, State Department spokesman John Kirby said.

"Our purchase of the heavy water means that it will instead be used for critically important research and non-nuclear industrial requirements," Kirby added.

The purchase, first reported by the Wall Street Journal, was slammed by Speaker of the House Paul Ryan, a Republican. Ryan said in a statement it appeared to be part of the Democratic administration's efforts to sweeten the nuclear deal with Iran and would "directly subsidize Iran's nuclear program."

The DOE expects to resell the purchased heavy water to domestic commercial and research buyers, including a national lab.-Reuters