Iran’s stockpile of near-weapons-grade nuclear fuel has jumped significantly, according to a confidential report by the United Nations atomic agency that found Tehran has almost doubled its output of highly enriched uranium in recent months, putting it closer to being able to produce the fuel needed for a nuclear weapon.

Iran had a stockpile of 17.7 kilograms (39 pounds) of 60% enriched uranium—its highest purity output so far—in early November, up from 10 kilograms in late August, meaning it is getting closer to having enough highly enriched nuclear fuel for a weapon.

The International Atomic Energy Agency sent the findings to member states on Wednesday. The reports come ahead of the resumption of negotiations in Vienna this month on reviving the 2015 nuclear deal, a top Biden administration foreign-policy goal.

The IAEA also reported for the first time what it called “excessively invasive physical searches” by Iran of the agency’s inspectors, describing months of exchanges with Tehran about the matter. The Wall Street Journal reported in September that there had been several incidents of physical harassment of female inspectors by Iranian security guards

Experts say Iran would need around 15 to 25 kilograms of weapons-grade nuclear fuel for a weapon. The current stockpile of 60% enriched uranium would likely produce roughly 8 to 12 kilograms of 90% nuclear fuel—the purity needed for a nuclear weapon—depending in part on Iran’s efficiency in producing the material and the precise makeup of the current material.

It could take Iran just weeks to enrich its 60% material to 90%, experts say.

Iran says its nuclear program is for purely peaceful purposes.

The latest IAEA reports show Iran will go into the nuclear talks with its stockpile of enriched uranium over 11 times what was allowed under the 2015 deal, including a large amount of 20% and 60% enriched uranium. The higher the percentage or purity of enriched uranium, the easier it is to increase the concentration.

Tehran has recently resumed the manufacturing of centrifuge parts at a site that isn’t being monitored by the U.N. atomic agency, according to diplomats. The country is producing uranium metal, a material that can be used in the core of a weapon, and has significantly scaled back the IAEA’s monitoring of its activities by restricting access to inspectors.

Iran has also progressed in its ability to produce more-advanced centrifuges, diplomats say, speeding the time it takes to produce enriched uranium.