The Institute for Science and International Security, a Washington think tank, released satellite imagery on Wednesday that it said showed a new, deeply buried tunnel alongside an older one around Natanz, as well as a new security perimeter.

"I've been raising this issue repeatedly, and I will continue to do so," Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), told reporters on a visit to Washington.

Grossi, who visited Tehran last week, said that all countries need to inform the IAEA of intentions for facilities around nuclear sites but that Iran has a stance "unique in the world" that it does not need to inform the agency ahead of time.

"We're asking them, what is this for? They are telling us, it's none of your business," Grossi said.

Grossi said it "cannot be excluded" that the tunnels would store undeclared material but said he did not want to speculate on intentions.

"Iran does not have nuclear weapons. This is very clear. We cannot confuse people. Iran has conducted in the past certain activities that were relevant -- could be relevant -- in the process of developing nuclear weapons," he said.

Grossi said he has heard optimism about a deal between the United States and Iran on the nuclear program after the arch-enemies held two rounds of talks, with technical talks expected this weekend.

"I think there's a general expectation that this goes well and that the agreement is verified by the IAEA," Grossi said.

He said that was "more or less the sense of what I'm getting when I'm talking to leaders," mentioning specifically Chinese Foreign Minister Wang Yi as hopeful for a US-Iran deal.

President Donald Trump in 2018 ripped up an earlier nuclear agreement negotiated under Barack Obama.

But he has voiced hope in his second term for a new accord that would resolve the issue diplomatically and has discouraged Israel from a military strike on Iran.

Asked about a military option, Grossi said: "I should simply remind that attacks on nuclear facilities is something that could have potentially very, very serious consequences."