The United States will buy 32 tonnes of heavy water from Iran to help it scale back its nuclear program, the State Department said Friday.

"This transaction provides US industry with a critical product, while also enabling Iran to sell some of its excess heavy water," spokesman John Kirby said.

Iran has agreed to scale back its nuclear program and place it under international inspection as part of an accord with world powers it implemented in January.

In return, it is supposed to receive sanctions relief, but tens of billions of dollars in frozen Iranian funds are still stuck in foreign banks.

US Secretary of State John Kerry was to meet his Iranian counterpart Mohammad Javad Zarif in New York later Friday to discuss ways to move forward.

The State Department was not able to say how much Washington will pay for the Iranian heavy water, which is produced as part of the nuclear energy process.

But Kirby confirmed that the purchase was designed to help Iran meet its obligations under the accord implementation process, known as the "JCPOA."

"Iran's compliance with the JCPOA meant that this material had already been removed from Iran, ensuring that it would not be used to support the development of a nuclear weapon," he said.

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

"We expect the heavy water to be delivered to the US in the coming weeks, initially stored at the Oak Ridge National Laboratory, and then resold at commercially reasonable prices to domestic commercial and research buyers."