PRESIDENT Hassan Rouhani said Tuesday that Iran would resume uranium enrichment at an underground plant south of Tehran in its latest step back from a troubled 2015 agreement with major powers.

The suspension of all enrichment at the Fordow plant in the mountains near the Shiite holy city of Qom was one of the main curbs on its nuclear activities that Iran accepted in return for the lifting of international sanctions.

But Washington’s abandonment of the deal in May last year followed by its reimposition of crippling sanctions prompted Iran to begin a phased suspension of its own commitments.

Under the terms of the agreement, Iran has retained more than 1,000 first-generation centrifuges at the Fordow plant, which have been running empty or remained idle since it took effect.

“Starting from tomorrow [Wednesday], we will begin injecting [uranium hexafluoride] gas at Fordow,” Rouhani said in a speech broadcast by state television.

His announcement came a day after tensions flared anew on the 40th anniversary of the U.S. embassy siege and hostage crisis, with thousands in Tehran taking to the streets and Washington imposing fresh sanctions.

The United States replied by vowing to keep up its “maximum pressure” campaign against Iran with a goal of ending all enrichment.

“Iran has no credible reason to expand its uranium enrichment program, at the Fordow facility or elsewhere, other than a clear attempt at nuclear extortion that will only deepen its political and economic isolation,” U.S. State Department spokeswoman Morgan Ortagus said.

Iran said the resumption of enrichment at Fordow would be carried out transparently and witnessed by inspectors from the United Nations nuclear watchdog, the International Atomic Energy Agency. (SD-Agencies)