Iran has significantly increased its stockpile of enriched uranium in recent months, continuing its nuclear escalation, a confidential report by the UN nuclear watchdog on Wednesday said.

The agency, however, noted progress in its cooperation with Iran and has decided to close the file on the presence of nuclear material at one of three undeclared sites, an issue which has poisoned relations between the two parties.

The reports came days before the board of governors of the International Atomic Energy Agency (IAEA) is due to meet to review progress in addressing the watchdog's remaining concerns.

The nuclear watchdog said in its report that Iran's estimated stockpile of enriched uranium had reached more than 23 times the limit set out in the landmark 2015 accord between Tehran and world powers.

As of 13 May, Iran's total enriched uranium stockpile was estimated at 4,744.5 kilogrammes. The limit in the 2015 deal was 202.8 kilograms.

The report also said that Iran is continuing its enrichment of uranium to levels higher than the 3.67 percent limit in the deal.

Efforts to revive the Iran nuclear deal – which was left in tatters by the unilateral withdrawal of the United States in 2018 – have currently stalled.

The stockpile of uranium enriched up to 20 percent is now believed to be 470.9 kilogrammes – up 36.2 kg since the last report in February – while the amount enriched up to 60 percent stands at 114.1 kilogrammes, an increase of 26.6 kg.

Enrichment levels of around 90 percent are required for use in a nuclear weapon.

In a separate report, the IAEA said it has decided to close the file relating to the presence of nuclear material at one undeclared site after receiving a "possible explanation" from Iran.

The watchdog "has no additional questions... and the matter is no longer outstanding at this stage", the report said of the site at Marivan in Abedeh county.

The IAEA had reported the discovery of traces of radioactive material at three sites not declared by Iran, in a blow to efforts to restore the 2015 deal.

The Marivan site in the southern province of Fars is the first to be addressed under a work plan agreed by Iran and the IAEA in March.

The other two sites are Varamin and Turquzabad.

Iran has always denied any ambition to develop a nuclear weapons capability, insisting its activities are entirely peaceful. (AFP)