Iran has started work on uranium fuel for a research reactor in the capital of Tehran, the U.N. nuclear watchdog said on Wednesday, constituting the latest violation of the nuclear deal.

News of the metal-based fuel production was confirmed by the state-affiliated Tasnim news agency.

The move is part of a wide-sweeping campaign to avenge the November killing of Mohsen Fakhrizadeh, the mastermind behind the country’s nuclear weapons program, which Tehran blamed on Israel as well as in response to sanction imposed by the U.S.

President-elect Joe Biden has promised to reenter the nuclear deal, from which President Donald Trump withdrew in 2018, after entering office later this month.

“(International Atomic Energy Agency) Director General Rafael Mariano Grossi today informed IAEA Member States about recent developments regarding Iran’s plans to conduct R&D activities on uranium metal production as part of its declared aim to design an improved type of fuel for the Tehran Research Reactor,” the IAEA said in a statement.

Earlier this month, Iran announced its plans to enrich uranium to 20 percent at its underground Fordo site, a move Israel has warned proves the Islamic Republic is seeking nuclear weapons.

E.U. foreign policy chief Josep Borrell on Monday warned Tehran its decision to enrich uranium was “a very serious development” with “potentially severe proliferation implications.”