Iran plans to install hundreds more advanced uranium-enriching centrifuges at an underground plant in breach of its deal with major powers, a UN nuclear watchdog report showed Friday.

The confidential International Atomic Energy Agency report obtained by Reuters said Iran plans to install three more cascades, or clusters, of advanced IR-2m centrifuges in the underground plant at Natan, 300 kilometers south of Tehran.

According to Iran's nuclear deal with major powers, Tehran can only use first-generation IR-1 centrifuges, which are less efficient, at the underground plant and that those are the only machines with which Iran may accumulate enriched uranium.

Iran recently moved one cascade of 174 IR-2m machines underground at Natanz and is enriching with it. It already planned to install two more cascades of other advanced models there, in addition to the 5,060 IR-1 machines that have been enriching for years in the plant built for more than 50,000.

"In a letter dated December 2, 2020, Iran informed the agency that the operator of the Fuel Enrichment Plant (FEP) at Natanz 'intends to start installation of three cascades of IR-2m centrifuge machines' at FEP," the report to its member states said.

Iran transferred the already-operating cascade of IR-2ms underground from an above-ground plant at Natanz where only a handful of those machines remain, the IAEA has said. 

The extra cascades would therefore have to involve some of the hundreds of IR-2m machines removed and put into storage under the 2015 deal.

The IAEA's last quarterly report on Iran last month showed Tehran had stockpiled 12 times the 202.8 kilograms of enriched uranium it is allowed to have under the deal, more than 2.4 tonnes.

That is still a fraction of the more than eight tonnes it had before the landmark 2015 deal, and it has not enriched uranium to a purity of more than 4.5 percent since then. It achieved 20 percent before 2015, closer to the 90 percent of weapons-grade uranium.

Tensions have been high between Tehran and Washington since 2018, when U.S. President Donald Trump exited Iran's 2015 nuclear deal and reimposed sanctions that have hit Iran's economy hard. In retaliation, Tehran has gradually breached the deal's curbs on its nuclear program.

The killing of one of Iran's top nuclear scientists Mohsen Fakhrizadeh on November 27 further heightened the tension. Iran believes Israel and an exiled opposition group carried out this assassination and vowed revenge for the killing though nobody has claimed responsibility so far.

(With input from Reuters)