DUBAI, United Arab Emirates (AP) — Iran has begun construction at its Natanz nuclear facility, satellite images released Wednesday show, just as the U.N. nuclear agency acknowledged Tehran is building an underground advanced centrifuge assembly plant after its last one exploded in a reported sabotage attack last summer.

The construction comes as the U.S. nears Election Day in a campaign pitting President Donald Trump, whose maximum pressure campaign against Iran has led Tehran to abandon all limits on its atomic program, and Joe Biden, who has expressed a willingness to return to the accord. The outcome of the vote likely will decide which approach America takes. Heightened tensions between Iran and the U.S. nearly ignited a war at the start of the year.

Since August, Iran has built a new or regraded road to the south of Natanz toward what analysts believe is a former firing range for security forces at the enrichment facility, images from San Francisco-based Planet Labs Inc. show.

A Planet Labs satellite image Monday shows the site cleared away with what appears to be construction equipment there, while an Oct. 21 image from Maxar Technologies shows trucks, cars, backhoes and other vehicles at the cleared site.

Analysts from the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies say they believe the site is undergoing excavation.

“That road also goes into the mountains so it may be the fact that they’re digging some kind of structure that’s going to be out in front and that there’s going to be a tunnel in the mountains,” said Jeffrey Lewis, an expert at the institute who studies Iran’s nuclear program. “Or maybe that they’re just going to bury it there.”

Rafael Grossi, the director-general of the International Atomic Energy Agency, told The Associated Press on Tuesday that his inspectors were aware of the construction. He said Iran had previously informed IAEA inspectors, who continue to have access to Iran’s sites despite the country having moved away from many limits of its landmark 2015 nuclear deal with world powers, known as the Joint Comprehensive Plan of Action, or JCPOA.

“They have started, but it’s not completed. It’s a long process,” Grossi said.

Alireza Miryousefi, a spokesman for the Iranian mission to the United Nations, would not comment on the satellite images or discuss specifics of the construction, but said Iran was being transparent with its actions.

“Nothing in Iran regarding its peaceful nuclear program is being done in secret, in full keeping with the JCPOA, and as the IAEA has repeatedly confirmed,” Miryousefi said in an email.

“This instance is no different,” he said.

Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran, last month told state television the destroyed above-ground facility was being replaced with one “in the heart of the mountains around Natanz.”

Trump in 2018 unilaterally withdrew the U.S. from the JCPOA deal Iran, in which Tehran agreed to limit its uranium enrichment in exchange for the lifting of economic sanctions. When the U.S. ramped up sanctions, Iran gradually and publicly abandoned those limits as a series of escalating incidents pushed the two countries to the brink of war at the beginning of the year.

Iran now enriches uranium to up to 4.5% purity, and according to the last IAEA report, had a stockpile of 2,105 kilograms. Experts typically say 1,050 kilograms of low-enriched uranium is enough material to be re-enriched up to weapons-grade levels of 90% purity for one nuclear weapon.