Iranian nuclear officials say they have switched on a modified part of the Arak heavy-water reactor in a step that could reportedly take the country closer toward weapons-grade levels of plutonium but does not breach its four-year-old nuclear deal with world powers.
It marks a further reactivation of Iran's nuclear program since the United States abandoned the Joint Comprehensive Plan of Action (JCPOA) last year.
Tehran agreed to shut down Arak under the 2015 agreement -- which swapped nuclear curbs for sanctions relief.
But it was allowed to renovate Arak and make a limited amount of heavy water, which slows down reactions in the core of nuclear reactors and produces plutonium as a byproduct.
Iran has taken limited steps toward leaving the JCPOA while pressing its non-U.S. signatories to find ways to help it avoid Washington's renewed sanctions, which have slammed Iran's oil sales, its economy, and its currency.
"Today we are...starting a noteworthy section of the reactor," the head of Iran's Atomic Energy Organization, Ali Akbar Salehi, said on Iranian television after the December 23 unveiling, according to Reuters.
Officials say the fully renovated reactor at Arak will be ready for initial tests in early 2021.
It will produce medical and agricultural isotopes, they say.
Tensions in the Middle East have escalated since U.S. President Donald Trump pulled out of the nuclear deal and reimposed tough punitive measures on Iran, with a number of naval confrontations and military incidents exacerbating fears of a serious conflict.
The Trump administration has signaled a desire to get Iranian officials back to the negotiating table, but Tehran has vowed not to do so until all sanctions are lifted.