“The position of the U.S. is that we’ve had concerns about Iran’s intentions with regard to its nuclear activity,” State Department spokeswoman Victoria Nuland said on Monday.

“We are calling on Iran to participate with the IAEA [International Atomic Energy Agency] to make the kind of transparent, open access to all of its facilities that would reassure all of us with regard to its intentions,” she told reporters.

“It claims its facilities are peaceful; if they’re purely peaceful, then there ought to be no difficulty allowing full inspection thereof,” Nuland said.

“We have concerns about some of the activity that we know about and why it would be ongoing if, in fact, the program is purely peaceful, because some of the things we’ve seen are inconsistent with a peaceful program,” she said.

Western nations suspect Iran of pursuing a secret nuclear weapons program but Tehran insists it needs nuclear power solely for civilian purposes.

Iran is currently subject to a series of EU and U.S. sanctions, including measures targeting the country's lucrative oil industry.