"Discussions are still ongoing" to determine the origin of these particles, the International Atomic Energy Agency (IAEA) said in a report seen by AFP.

Asked about the particles found in Iran, the government in Tehran said "unintended fluctuations" during the enrichment process "may have occurred".

In 2015, Iran reached a deal with world powers to limit the enrichment of uranium and allow IAEA inspectors to visit its nuclear sites, in return for the lifting of economic sanctions.

But the deal stalled in 2018.

Iran has been enriching uranium well over the limits laid down in the deal and the IAEA believes its estimated stockpile is more than 18 times the limit set out in that accord.

The IAEA report said that during an inspection "on 22 January 2023, the agency took environmental samples... at the Fordow (sic) plant, the analytical results of which showed the presence of high enriched uranium particles containing up to 83.7 percent U-235".

"These events clearly indicate the capability of the agency to detect and report in a timely manner changes in the operation of nuclear facilities in Iran," it continued.

In the report, the United Nations' nuclear watchdog also said that Iran's estimated stockpile of enriched uranium had reached more than 18 times the limit set out in the 2015 accord between Tehran and world powers.

The IAEA estimated Iran's total enriched uranium stockpile was 3,760.8 kilogrammes (8,291 pounds) as of February 12, an increase of 87.1 kilogrammes compared to the last report in November.

The limit in the 2015 deal was set at 202.8 kilogrammes of uranium.

The IAEA has also repeatedly warned it has lost its ability to fully monitor Iran's programme since the Islamic republic started to restrict its access in February 2021.

Iran has been enriching uranium well over the limits laid down in the landmark accord, which started to unravel when the United States withdrew from it in 2018.

Talks between Tehran and world powers have stalled to revive the deal, which gave Iran much-needed sanctions relief in return for curbs on its atomic programme.

Iran's stockpile of uranium enriched to 60 percent is now at 87.5 kilogrammes, up from 62.3 kilogrammes, the report said.

Iran now also has 434.7 kilogrammes of uranium enriched up to 20 percent, up from 386.4 kilogrammes in the November report.