DUBAI, United Arab Emirates -- Iran has begun construction on a site at its underground nuclear facility at Fordo as tensions escalate with the U.S. over its atomic program, according to satellite photos obtained Friday by The Associated Press.
Iran has not publicly acknowledged any new construction at Fordo, whose discovery by the West in 2009 was made in an earlier round of brinkmanship before world powers struck the 2015 nuclear deal with Tehran.
While the purpose of the building remains unclear, any work at Fordo is likely to trigger new concern in the waning days of the Trump administration before the inauguration of President-elect Joe Biden. Already, Iran is building at its Natanz nuclear facility after a mysterious explosion there in July that Tehran described as sabotage.
"Any changes at this site will be carefully watched as a sign of where Iran's nuclear program is headed," said Jeffrey Lewis, an expert at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies who studies Iran.
Asked for comment, Iran's mission to the United Nations said "none of Iran's nuclear activities are secret," given the ongoing inspections by the International Atomic Energy Agency.
"We have always maintained that our current activities, which are in line with [the nuclear deal], can and will be immediately reversed once the other parties, including the U.S., come into full compliance with what was agreed upon, in particular on removing sanctions," mission spokesman Alireza Miryousefi said. He did not elaborate.
The Vienna-based atomic energy agency, whose inspectors are in Iran as part of the nuclear deal, declined to comment. The agency has not publicly disclosed if Iran informed it of any construction at Fordo.
Construction on the Fordo site began in late September. Satellite images obtained from Maxar Technologies by the AP show the construction taking place at a northwest corner of the site, near the holy Shiite city of Qom, some 55 miles southwest of Tehran.
A Dec. 11 satellite photo shows what appears to be a dug foundation for a building with dozens of pillars. Such pillars can be used in construction to support buildings in earthquake zones.
The construction site sits northwest of Fordo's underground facility, built deep inside a mountain to protect it from potential airstrikes. The site is near other above-ground support and research-and-development buildings at Fordo.
Among those buildings is Iran's National Vacuum Technology Center. Vacuum technology is a crucial component of Iran's uranium-gas centrifuges, which enrich uranium.
A Twitter account called Observer IL this week published an image of Fordo showing the construction, citing it as coming from South Korea's Korea Aerospace Research Institute.
The AP later reached the Twitter user, who identified himself as a retired Israeli soldier with a civil engineering background. He asked that his name not be published over previous threats he received online. The Korea Aerospace Research Institute acknowledged taking the satellite photo.
President Donald Trump in 2018 unilaterally withdrew the U.S. from Iran's nuclear deal, in which Tehran had agreed to limit its uranium enrichment in exchange for the lifting of economic sanctions. Trump cited Iran's ballistic missile program, its regional policies and other issues in withdrawing from the accord, though the deal focused entirely on Tehran's atomic program.
When the U.S. ramped up sanctions, Iran gradually and publicly abandoned the deal's limits as a series of escalating incidents pushed the two countries to the brink of war at the beginning of the year. Tensions remain high.
Under the 2015 nuclear deal, Iran agreed to stop enriching uranium at Fordo and instead make it "a nuclear, physics and technology center."
"This location was a major sticking point in negotiations leading to the Iran nuclear deal," Lewis said. "The U.S. insisted Iran close it while Iran's supreme leader said keeping it was a red line."
Since the deal's collapse, Iran has resumed enrichment there.
