The United Nations's nuclear watchdog, the International Atomic Energy Agency (IAEA) expressed concern about Iran's increased production of highly enriched uranium over the past three months.
Between the end of October and the beginning of February, 25 kilograms of almost weapons-grade uranium with a purity of 60% was produced, the U.N.'s nuclear watchdog stated in a report on Monday. In the previous quarter, the figure was less than 7 kilograms.
However, Iran's total stockpile of 60%-enriched uranium fell slightly to 121.5 kilograms in recent months, according to the report, because the material was mixed with lower-grade enriched uranium.
According to experts, however, this mixture could be brought back to a weapons-grade level. Around 50 kilograms of at least 80 to 90%-enriched uranium is required for a nuclear bomb.
Iran's leadership denies wanting to build nuclear weapons. However, in a further report on Monday, IAEA Director General Rafael Grossi pointed to recent statements by the current and former Iranian nuclear chiefs that their country has the technical capabilities to develop such weapons.
Such statements only increase the director general's concerns as to whether Iran has disclosed its entire nuclear program, the IAEA said.
Tensions between Iran and IAEA have repeatedly flared up since a 2015 deal curbing Tehran's nuclear program in exchange for sanction relief fell apart.
In recent years, Iran has gradually decreased its cooperation with the IAEA by deactivating surveillance devices needed to monitor the nuclear program and barring inspectors among other measures.
Grossi reiterated his call on Tehran to "cooperate fully and unambiguously with the agency," as relations between the two parties have been steadily deteriorating.
"Only through constructive and meaningful engagement can these concerns be addressed," Grossi said in a confidential quarterly report.
While Tehran denies seeking to acquire nuclear weapons, some politicians and officials have made concerning statements about the country's technical capabilities, a diplomatic source said.
At the same time, Iran has significantly ramped up its nuclear program and now has enough material to build several atomic bombs.
In a separate confidential report seen by AFP ahead of an IAEA board of governors' meeting next week, the agency said that Iran's estimated stockpile of enriched uranium had reached 27 times the limit set out in the 2015 accord.
EU-mediated efforts to revive the deal – bringing the U.S. back on board and Iran back into compliance – have so far been fruitless.
Grossi also "deeply regrets" that Iran has not yet reversed its decision to ban several of its inspectors.
Iran in September withdrew the accreditation of several inspectors, a move Teheran described as retaliation for "political abuses" by the United States, France, Germany and Britain.
The IAEA has condemned the move – which targets eight top inspectors, with French and German nationals among them, according to a diplomatic source.
Iran's "unprecedented" move has "directly and seriously affected" the U.N. body's work.
Faced with increased criticism, the Iranian government announced last week that it had invited Grossi to come to Tehran in May for an international conference on energy.