"The process for producing 20 percent enriched uranium has started at Shahid Alimohammadi enrichment complex (Fordow)," government spokesman Ali Rabiei said, quoted on the website of the state broadcaster.

According to the official, President Hassan Rouhani ordered the enrichment "in recent days", and "the gas injection process started as of hours ago."

The move follows a bill passed last month by the conservative-dominated parliament "for the lifting of sanctions and protection of the Iranian people's interests" and mandating Rouhani's government to "produce and store 120 kilogrammes (265 pounds) per year of uranium enriched to 20 percent".

The law also calls on the administration to end UN inspections of Iran's nuclear facilities, if the remaining parties to the deal -- Britain, China, France, Germany and Russia -- do not facilitate Iran's oil sales and guarantee the return of the proceeds.

Before the bill became law, Rouhani slammed it as "detrimental to the course of diplomatic activities."

But the Guardian council, which arbitrates disputes between the parliament and the government, approved the bill into law last month.

Iranian officials, including foreign minister Javad Zarif, had said the government will comply with the parliament's decision.

Quoted by the government's website, Rabiei said that the administration's stance towards the law is clear, "but the government considers itself bound to carry out the law."

The government has signalled a readiness to engage with US President-elect Joe Biden after four tense years under Trump, who reimposed and reinforced crippling sanctions on Tehran after withdrawing the US from the nuclear agreement.