Iran resumed operations of uranium enrichment up to 20% at an underground nuclear facility after a pause under the 2015 agreement.

"A few minutes ago, the production of 20% enriched uranium began at the Fordow complex," Iranian government spokesman Ali Rabiei said on television on January 5.

The United Nations Agency for International Atomic Energy (IAEA) also confirmed that Iran has begun the process of enriching uranium to 20% purity at the Fordow facility.

"Iran today started bringing U-235 enrichment to 4.1% in six centrifuges at the Fordow Fuel Enrichment Plant to get rich up to 20%," the IAEA said in a statement.

This move is one of the things mentioned in the law passed by the Iranian parliament in December 2020 in response to the murder of Mohsen Fakhrizadeh, the country's top nuclear expert.

Iran's decision to enrich uranium at 20% is in violation of a nuclear agreement (JCPOA) signed by the country with major powers in 2015. The deal is to extend the time it takes Iran to produce enough material for a bomb.

However, Tehran said it could quickly reverse the breach of the agreement if the US sanctions were lifted.

Iran's move could complicate efforts to bring the US back to President-elect Joe Biden's deal.

The resumption of uranium enrichment comes amid heightened tensions between Iran and the United States in the last days of President Donald Trump in office.

A State Department spokesman accused Iran of "nuclear blackmail".

Katsunobu Kato, Chief Cabinet Secretary of Japan, said the country was "deeply concerned" about Iran's move and called it an act of "breaking the nuclear deal".

Iran previously broke its commitment to enrich uranium only to 3.67% under the 2015 agreement, but the Middle East only increased its pure uranium concentration by 4.5%, which is 20% lower than the arms grade.

US intelligence and the IAEA claim that Iran deployed a secret nuclear weapons program and stopped it in 2003. Iran has said that it has never implemented it.