President Donald Trump has heralded the peace talks with Iran as a win for US farmers, saying that the unfreezing of sanctioned Iranian money will be tied to that country buying American-grown corn, soybeans and wheat.

"These are things that are desperately needed by Iran," Trump posted on social media. "This is a humanitarian crisis, and I feel it is necessary to help."

But Iran is unlikely to start buying a vast amount of US farm products.

"I don't expect that trade would be very large in the short run," said Joseph Glauber, a research fellow emeritus at the International Food Policy Research Institute.

Glauber noted that Iran was "unlikely" to abandon its other trade partners on food for America. He said Iran's major suppliers include Brazil, India, Turkiye, the European Union, Canada, Australia and Argentina and that Trump's demand to buy from the US would "create some hard feelings with some of our competitors.