The US and its European allies are planning to complain to the UN nuclear chief that his comments that Iran should be allowed to keep some uranium enrichment were unhelpful, according to senior officials.
Diplomats said on Tuesday the US would inform Mohamed ElBaradei of their protest before he delivers his latest report on Iran.
"They have decided to do so. It will maybe be before the report comes out," a diplomat said, speaking on condition of anonymity.
A second diplomat said the head of the US mission to the International Atomic Energy Agency, Gregory Schulte, is expected, possibly along with ambassadors from US allies, "to speak to ElBaradei and complain".
Schulte's spokesman refused to comment.
ElBaradei, the head of the IAEA, has gone public with his view that it is too late to try to force Tehran to scrap its enrichment programme.
Differing strategies
"From a proliferation perspective, the fact of the matter is that one of the purposes of suspension [of uranium enrichment] - keeping them from getting the knowledge - has been overtaken by events," he told the New York Times earlier this month.
ElBaradei said: "The focus should be to stop them from going to industrial scale production," rather than expecting the Iranians to stop all enrichment.
The US leads Western nations in insisting that Iran freeze all enrichment work in order to start talks on trade, security and technology benefits for Tehran in return for guarantees it will not seek nuclear weapons.
Iran is defying demands and sanctions from the UN Security Council for it to suspend uranium enrichment, which the West fears could be used to develop nuclear weapons.
Iran says it needs to enrich to produce fuel for nuclear reactors in order to eventually produce electricity.
Some diplomats have been talking about a possible compromise in which Iran would be allowed to continue research into enrichment, at a time of diplomatic initiatives by both the US and the European Union with Iran.