Iran is continuing to build up its stockpile of low-enriched uranium and now holds roughly 12 times the amount permitted under the 2015 nuclear agreement, the United Nations Atomic Agency said in a report. The report’s findings underscore the challenge the incoming Biden administration faces in persuading Iran to fully return to the 2015 nuclear deal: Besides the stockpile of low-enriched uranium, which when further refined can be used to fuel a nuclear weapon, Iran is also taking steps to potentially accelerate its production of low-enriched uranium and is continuing its nuclear research.

President-elect Joe Biden has said he is prepared to take the U.S. back into the 2015 nuclear deal provided Iran returns into full compliance with that deal and agrees to future negotiations for longer and more stringent constraints on its nuclear activities.

Mr. Biden, who was vice president when the Iran deal was struck, has criticized the Trump administration’s decision to quit the nuclear deal in May 2018 and impose sweeping sanctions on Tehran. In September, he said that approach “recklessly tossed away a policy that was working to keep America safe and replaced it with one that has worsened the threat.”

Iran has said it is open to negotiation but has placed various conditions on returning to the accord’s terms, including compensation for the U.S.’s withdrawal and sanctions. On Wednesday, Iran’s deputy foreign minister, Abbas Araghchi, was quoted in Iranian state media saying “the way back is open” for the U.S. to the deal.

Iran has gradually moved away from the nuclear deal’s limits since the summer of 2019 in response to the U.S.’s “maximum pressure” sanctions campaign.

Mr. Trump, in justifying his decision to take the U.S. out of the deal, said it would fail to stop Tehran from eventually obtaining nuclear weapons and ignored key issues, like Iran’s ballistic-missile capacity and its support for terrorism. Trump administration officials have indicated they could slap new sanctions on Iran in the final weeks before January’s transition.

In its latest quarterly report, sent to member states and seen by The Wall Street Journal, the International Atomic Energy Agency said Iran had now accumulated a low-enriched uranium stockpile of 2,443 kilograms. That compares with a limit of 203 kg under the nuclear deal.

Of the total, which the agency said is enriched up to 4.5% purity, around a quarter has been produced in a way that nuclear experts say would be of little use for further enrichment. Nonetheless, Iran has now accumulated enough enriched uranium to produce the high-enriched uranium needed for two nuclear weapons, according to analysts at the Institute for Science and International Security. Weapons-grade material is of around 90% purity.