Iranian President Hassan Rouhani visits the Bushehr nuclear power plant. (AP Photo)

Iran may be just one month away from amassing enough weapons-grade uranium to develop a nuclear weapon, according to analysis published on Monday by a US-based think tank.

The Institute for Science and International Security (ISIS) projects that in a worst-case scenario, Iran can produce enough uranium for one nuclear weapon "in as short as one month," and enough for a second weapon in "less than three months," and a third within five months.

This does not mean Iran is weeks away from firing a nuclear weapon. Iran must first use the uranium to assemble a weapon, install it on a nuclear warhead, and make sure it survives re-entry into the atmosphere - a process that takes much longer to complete. The Israel Defense forces said in February that it could take Iran two years to develop a deliverable nuclear bomb. Iran insists it has no desire to create such a weapon.

Monday's report is based on expert analysis of recent findings from the UN's nuclear watchdog - the International Atomic Energy Agency. The agency found that Iran has continued to increase its stockpile of enriched uranium in violation of the 2015 nuclear deal, which promises Iran economic incentives in exchange for limits on its nuclear program.

The IAEA told member states that its monitoring activities at Iran's nuclear sites have been "seriously undermined" since February by Tehran's refusal to let agency inspectors access monitoring equipment, including security cameras. The agency found a temporary solution on Sunday and struck a deal with Iran that will allow IAEA inspectors to continue filming the country's nuclear sites.

"Overall, the IAEA's latest report shows Iran's rapidly advancing nuclear activities and steps to limit IAEA monitoring, while inspectors have a diminishing ability to detect Iranian diversion of assets to undeclared facilities. The IAEA is sounding an alarm to the international community accordingly," ISIS said.

Israeli officials have echoed the think tank's warning.

Israeli Defense Minister Benny Gantz said in August that Iran is weeks away from acquiring enough material to make a nuclear bomb. He urged the international community to find a viable "Plan B" to stopping Iran's nuclear program if talks to restore the 2015 deal fail.

President Joe Biden has said he wants to restore or renegotiate the nuclear agreement (JCPOA) after former US President Donald Trump abandoned the deal in 2018, but talks have stalled. Iran responded to the US withdrawal by abandoning most of its commitments to the deal, arguing that if Washington won't abide by it, neither will Tehran.

Critics of the JCPOA said it failed to address Iran's ballistic missile program and its development of a nuclear warhead. Supporters of the agreement argue that it significantly decreased Iran's enrichment of uranium to 3.67% and delayed Iran's breakout time - the time it takes for it to stockpile enough weapons-grade material for a nuclear weapon.

While it might take years for Iran to create a deliverable nuclear bomb, it has never been this close to nuclear capability.

Secretary of State Antony Blinken told reporters on Friday that Iran's rapid nuclear progress could make the 2015 deal irrelevant.

"I'm not going to put a date on it but we are getting closer to the point at which a strict return to compliance" with the old deal "does not reproduce the benefits that that agreement achieved," he said.

Meanwhile, Israel has threatened to take military action to confront Iran's nuclear program.

"We have no choice but to expand our force build-up, to continue to rely on our human capital, and to adapt our capabilities and our plans," Gantz said at a military ceremony in July.

"All of these threats demand that we speed up and increase our preparedness to carry out our mission with an iron wall of action and not to get by with just words," Gantz said.