The standoff on Iran's nuclear programme is worrying, the head of the UN atomic watchdog said Monday, but stressed that work for a diplomatic solution should continue.

Yakio Amano's comments came as the International Atomic Energy Agency (IAEA) warned that Tehran was on the cusp of being able to triple output of nuclear material that, if further treated, could be used in the core of a bomb.

"The situation is worrying but it is important to continue to seek a diplomatic solution," Amano told reporters after talks with French President Francois Hollande.

Iran's nuclear drive is a growing international concern as Israel has given indications that it is ready to stage a military strike. US President Barack Obama has said he will not let Iran get a nuclear bomb.

Amano said the "current situation is worrying but the declared material and installation facilities are under the IAEA safeguard and we can verify that they stay in a peaceful purpose.

"We are going to have a high-level dialogue with Iran on the 13th of December in Tehran. The way to solve this issue is by diplomatic means and we will continue our efforts," he said.

The UN Security Council has passed four rounds of nuclear sanctions against Iran, and the IAEA's board of governors passed a resolution in September again calling on Iran to prove its programme has no military dimension.