"Iran today began feeding uranium already enriched up to 4.1 percent U-235 into six centrifuge cascades at the Fordow Fuel Enrichment Plant for further enrichment up to 20 percent," a statement from the International Atomic Energy Agency (IAEA) said.

The statement added that "IAEA inspectors were present at the site to detach the Agency's seal from a cylinder with the feed material" and that "the cylinder was then connected to the feeding line to start the production of uranium enriched up to 20 percent".

A total of 1,044 IR-1 centrifuges were being used in the process, the IAEA said.

Iran had announced earlier on Monday that it was starting the process to enrich uranium to 20 percent, the latest and most important step away from the 2015 deal that Tehran has yet taken.

Iran started violating the limits on nuclear activity set down in the deal in 2019 in retaliation for the US President Donald Trump's withdrawal from the accord the previous year and subsequent imposition of harsh economic sanctions.