VIENNA -- The U.N. atomic watchdog said Thursday it believes that Iran has further increased its stockpile of highly enriched uranium and criticized Tehran for continuing to bar the agency's officials from accessing or monitoring Iranian nuclear sites.

In its quarterly report, the International Atomic Energy Agency said that according to its assessment, as of Oct. 22, Iran has an estimated 62.3 kilograms (137.3 pounds) of uranium enriched to up to 60% fissile purity. That amounts to an increase of 6.7 kilograms since the IAEA's last report in September.

That enrichment to 60% purity is one short, technical step away from weapons-grade levels of 90%. Nonproliferation experts have warned in recent months that Iran now has enough 60%-enriched uranium to reprocess into fuel for at least one nuclear bomb.

The IAEA report, which was seen by The Associated Press, also estimated that as of Oct. 22, Iran's stockpile of all enriched uranium was at 3673.7 kilograms -- a decrease of 267.2 kilograms since the last quarterly report in September.

The Vienna-based IAEA said it was unable to verify the exact size of Iran's stockpile of enriched uranium due to limitations that Tehran imposed on U.N. inspectors last year and the removal of the agency's monitoring and surveillance equipment in June at sites in Iran.

It has been nearly two years since IAEA officials have had full access to monitor Iran's nuclear sites, and five months since the surveillance equipment was removed.

The IAEA's assessment comes as efforts to revive Iran's 2015 nuclear deal with world powers, which eased sanctions on Iran in return for curbs on its nuclear program, have stalled.

The United States unilaterally pulled out of the nuclear deal -- formally known as the Joint Comprehensive Plan of Action, or JCPOA -- in 2018, under then-President Donald Trump. It reimposed sanctions on Iran, prompting Tehran to start backing away from the deal's terms.

The IAEA said in its report that the lack of cooperation from Iran would have a "significant impact" on the agency's ability to reestablish its knowledge of Iran's activities since its cameras were removed in June.

"Any future baseline for the ... JCPOA verification and monitoring activities would take a considerable time to establish and would have a degree of uncertainty," the report stated. "The longer the current situation persists, the greater such uncertainty becomes."

A separate report, also seen by the AP, said IAEA Director General Rafael Grossi is "seriously concerned" that Iran has still not engaged on the agency's probe into man-made uranium particles found at three undeclared sites in the country. The issue has become a key sticking point in the talks for a renewed nuclear deal.

Grossi met with Mohammad Eslami, vice president and head of Iran's Atomic Energy Organization, in late September to discuss the topic. The second report on Thursday noted that IAEA officials will travel to Tehran for a technical visit by the end of November.

That meeting, the IAEA report said, "should be aimed at effectively clarifying and resolving" remaining safeguards issues.

The IAEA has for years sought answers from Iran to its questions about the particles. U.S. intelligence agencies, Western nations and the IAEA have said Iran ran an organized nuclear weapons program until 2003. Iran has long denied ever seeking nuclear weapons, insisting its nuclear program is peaceful.