Iran says it will only sign up to a final nuclear deal, if all sanctions imposed on the country are lifted and all at the same time.
It comes after a preliminary agreement was struck in Switzerland last week, which aims to curb Tehran’s disputed atomic work.
“We will not sign any deal unless all economic sanctions are immediately lifted on the same day,” said president Hassan Rouhani.
But his words are at odds with the US, which says the sanctions would have to be phased out gradually.
The White House is in the midst of trying to persuade lawmakers and critics at home that the initial deal struck in Switzerland is the right one.
Secretary of State John Kerry is making it clear that Iran must reveal details about its nuclear programme.
“They have to do it. It will be done. If there’s going to be a deal, it will be done,” he said.
The deadline for reaching a final nuclear deal is the end of June and there are still many details to work out.
The world powers involved are expected to meet in the coming days to try to move things forward.