The head of the UN's nuclear watchdog has given his clearest signal yet that inspectors will gain access to Iran's uranium enrichment sites, pushing back against contradictory statements from both Washington and Tehran that have muddied the terms of last week's landmark ceasefire deal.

Speaking at a press conference at Japan's Fukushima Daiichi nuclear power plant on Wednesday, International Atomic Energy Agency Director General Rafael Mariano Grossi said the memorandum of understanding signed by both presidents leaves no room for ambiguity.

"I can understand political statements, they are part of the reality, but the fundamental thing I would like to remind you and draw your attention to is that there has been a Memorandum of Understanding, signed by both presidents," Grossi told journalists.

The accord, he said, "says explicitly that the nuclear activities that are going to be carried out with regards to the nuclear material facilities will be supervised by the IAEA — in all letters."

"Obviously, to do that, we will have to inspect. Whether this happens the day after tomorrow or in one week or in 10 days, it's important, but not essential. This is going to happen," he added.