The head of the UN nuclear watchdog agency called Wednesday on Iran to explain tunnels built around a nuclear site but voiced optimism that US-Iran talks would land a deal.

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 the Natanz site, 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........

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