Iran has "increased its production of highly enriched uranium, reversing a previous output reduction from mid-2023", the International Atomic Energy Agency said in a statement.

Iran has increased its production of 60 percent enriched uranium to a rate of about 9 kilogrammes (20 pounds) a month since the end of November. That's up from about 3 kilogrammes a month since June, and a return to the 9 kilogrammes a month it was producing during the first half of 2023, the IAEA said.

"On 19 and 24 December, IAEA inspectors verified the rate of production of uranium enriched to this level at the two facilities where Iran is carrying out these activities -- the Natanz Pilot Fuel Enrichment Plant and the Fordow Fuel Enrichment Plant," it said.

Nuclear weapons require uranium enriched to 90 percent, while 3.67 percent is enough for nuclear power stations.

Iran appeared to have slowed its enrichment as a gesture while informal talks for a nuclear treaty had resumed with the United States.

But animosity between the two countries has intensified in recent months, with each one accusing the other of exacerbating the war between Israel and Hamas.

In November, a confidential IAEA report seen by AFP indicated that Iran's enriched uranium stocks were 22 times the limits authorised in the 2015 accord limiting Iran's nuclear activities in exchange for lifting sanctions.

That accord fell apart in 2018 when then president Donald Trump pulled out the United States.

His successor Joe Biden has tried to revive the accord through talks in Vienna, but the process has been at a standstill since the summer of 2022.

Iran, which has supported Russia's invasion of Ukraine, has prevented IAEA inspections and disconnected surveillance cameras installed at its nuclear programme sites.

In November, it held 567.1 kilogrammes of uranium enriched at 20 percent and 128.3 kilogrammes at 60 percent, three times what would be needed to build an atomic bomb if enriched to 90 percent.