France on Thursday called developments in Iran's nuclear programme "very concerning" after the UN nuclear watchdog reported finding uranium particles enriched just under the 90 percent needed for an atomic bomb.

"This report states that the direction Iran is taking is very concerning," foreign ministry spokeswoman Anne-Claire Legendre told reporters, adding this development was "unprecedented and extremely serious".

The International Atomic Energy Agency (IAEA) confirmed Tuesday that it had detected particles of uranium enriched to up to 83.7 percent, only just short of the 90 percent needed to produce a nuclear device.

Last week, Iran claimed it had not made any attempt to enrich uranium beyond 60 percent.

The head of the IAEA is to meet with Iran's President Ebrahim Raisi in Tehran on Saturday to try to "relaunch the dialogue" on the country's atomic work, a diplomatic source said Wednesday.

Iran has been enriching uranium well over the limits laid down in a landmark 2015 deal with world powers, which started to unravel when the United States withdrew from it in 2018.