■ Construction is taking place at an underground nuclear facility in Iran.

DUBAI, United Arab Emirates — Iran has begun construction on a site at its underground nuclear facility at Fordo amid tensions with the U.S. over its atomic program, according to satellite photos.

Iran has not publicly acknowledged any new construction at Fordo, whose discovery by the West in 2009 came 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 likely will trigger new concern in the waning days of the Trump administration before the inauguration of President-elect Joe Biden. Iran is already building at its Natanz nuclear facility after a mysterious explosion in July there that Tehran described as a sabotage attack.

“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 that “none of Iran’s nuclear activities are secret,” given the ongoing inspections by the International Atomic Energy Agency.

The Vienna-based IAEA, whose inspectors are in Iran as part of the nuclear deal, declined to comment. As of yet, the agency has not publicly disclosed whether Iran informed it of any construction at Fordo.

Construction on the Fordo site began in late September.

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.