Tensions between Iran and the UN nuclear watchdog have repeatedly flared since a 2015 deal curbing Tehran's nuclear programme in exchange for sanctions relief fell apart.

The IAEA said in its quarterly report that it is of "serious concern" that Iran has an estimated 274.8 kilograms (605 pounds) of uranium enriched to up to 60 percent as of 8 February, up by 92.5 kilograms since the previous report in November.

That level of enrichment is a short step from the 90 percent needed to make a nuclear weapon.

Tehran in December warned it would ramp up production of uranium enriched to 60 percent in response to the adoption of an IAEA board resolution that censured Tehran for its lack of cooperation with the agency.

Iran had also announced that it would launch "new and advanced" centrifuges.

Tehran insists on its right to nuclear energy for peaceful purposes and has denied it is seeking a weapons capability.

But according to the IAEA, Iran is the only non-nuclear weapon state to enrich uranium to 60 percent, which is higher than what's necessary to produce nuclear power.

"The significantly increased production and accumulation of high enriched uranium by Iran, the only non-nuclear weapon State to produce such nuclear material, is of serious concern," the confidential report said.

The rate of production of highly enriched uranium has jumped to between 35 to 40 kilograms per month, compared to around 6 kilograms previously, a diplomatic source told AFP.

- 'Comprehensive' assessment -

In recent years, Tehran has increasingly rolled back its cooperation with the IAEA by deactivating surveillance devices needed to monitor the nuclear programme, and barring UN inspectors.

UN nuclear watchdog head Rafael Grossi in a separate report said he would "produce a comprehensive and updated" report on Iran's nuclear programme, including on the "presence and use of undeclared nuclear material in connection with past and present outstanding issues".

As of early February, Iran's stockpile of uranium at various levels of enrichment was estimated at 8,294.4 kilograms, up by 1,690 kg since the previous quarterly report.

The amount is more than 40 times the limit set in the 2015 accord with world powers.

The two reports come on the eve of the IAEA board of governors meeting.

Efforts to revive the 2015 landmark nuclear deal -- bringing the United States back on board and Iran back into compliance -- have so far been fruitless.

Earlier this week, Iranian Foreign Minister Abbas Araghchi dismissed the possibility of direct negotiations with the United States on his country's nuclear programme after Washington announced fresh sanctions on Tehran.

Crisis Group analyst Ali Vaez said that Iran "will not negotiate with a gun to its head".

Donald Trump unilaterally withdrew the United States from a 2015 deal with Tehran during his first term as president. The deal had exchanged sanctions relief for limits on Iran's nuclear programme.