The report comes at a sensitive time as Tehran and Washington have been holding several rounds of talks over a possible nuclear deal that US President Donald Trump is trying to reach.
The report by the Vienna-based International Atomic Energy Agency - which was seen by The Associated Press - says that as of May 17, Iran has amassed 408.6 kilograms of uranium enriched up to 60 per cent.
That is an increase of 133.8km since the IAEA's last report in February.
That material is a short, technical step away from weapons-grade levels of 90 per cent.
A report in February put the stockpile at 274.8km.
The IAEA chief Rafael Mariano Grossi has stressed repeatedly that "Iran is the only non-nuclear weapon state enriching to this level".
On Saturday, Grossi said he "reiterates his urgent call upon Iran to co-operate fully and effectively" with the IAEA.
On Thursday, senior Iranian officials dismissed speculation about an imminent nuclear deal with the United States, emphasising that any agreement must fully lift sanctions and allow the country's nuclear program to continue.
The comments came a day after Trump said he has told Israeli Prime Minister Benjamin Netanyahu to hold off on striking Iran to give the US administration more time to push for a new nuclear deal with Tehran.
Trump said on Friday that he still thought a deal could be completed in the "not too distant future".
"They don't want to be blown up. They would rather make a deal," Trump said of Iran.
He added, "That would be a great thing that we could have a deal without bombs being dropped all over the Middle East."
Trump said he had warned Israeli Prime Minister Benjamin Netanyahu from carrying out pre-emptive strikes on Iran's nuclear program, while the talks were playing out.
