DUBAI, United Arab Emirates (AP) – 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 obtained by The Associated Press on Friday.

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. Already, Iran is 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 told the AP that “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.

Construction on the Fordo site began in late September. Satellite images obtained from Maxar Technologies by 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 December 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.

Under the 2015 nuclear deal with the U.S. and other world powers, Iran agreed to stop enriching uranium at Fordo and instead make it “a nuclear, physics and technology center.”

Shielded by the mountains, the facility also is ringed by anti-aircraft guns and other fortifications. It is about the size of a football field, large enough to house 3,000 centrifuges, but small and hardened enough to lead U.S. officials to suspect it had a military purpose when they exposed the site publicly in 2009.

As of now, Iran is enriching uranium up to 4.5 percent, in violation of the accord’s limit of 3.67 percent. Iran’s parliament has passed a bill that requires Tehran to enrich up to 20 percent, a short technical step away from weapons-grade levels of 90 percent. The bill also would throw out IAEA inspectors.

Experts say Iran now has enough low-enriched uranium stockpiled for at least two nuclear weapons, if it chose to pursue them. Iran long has maintained its nuclear program is peaceful.

While Iranian President Hassan Rouhani opposed the bill, the country’s Guardian Council later tweaked and approved it. The bill seeks to pressure European nations to provide relief from crippling U.S. sanctions.

Meanwhile, an Iranian scientist who created its military nuclear program two decades ago recently was killed in a shooting outside of Tehran. Iran has blamed Israel, which has long been suspected of killing Iranian nuclear scientists over the last decade, for the attack. Israel has not commented on the attack.