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's latest report comes as Tehran pursues delicate negotiations with the United States on its nuclear programme.

It said in its quarterly report that it is of "serious concern" that Iran has an estimated amount of 408.6 kilogrammes enriched to up to 60 percent as of May 17, up by 133.8 kilogrammes since the last report in February.

Uranium enriched to up to 60 percent fissile purity is close to the roughly 90 percent level needed for atomic weapons.

According to the IAEA, Tehran is the only non-nuclear weapon state to enrich uranium to 60 percent.

Iran has always denied seeking a nuclear weapon.

In a separate in-depth report, the IAEA criticised "less than satisfactory" cooperation from Tehran over its scrutiny of Iran's nuclear programme.

"While Iran continues to cooperate with the agency on matters of routine safeguards implementation, in a number of respects... its cooperation with the agency has been less than satisfactory," the report said.

It specifically notes Tehran's lack of progress in explaining nuclear material found at undeclared sites.

"In particular, Iran has repeatedly either not answered, or not provided technically credible answers to, the agency's questions and has sanitised locations as listed in this report, which has impeded agency verification activities."

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