Iran says it has resumed enriching uranium to 20-percent purity, a move that would significantly violate the 2015 nuclear deal with major powers.
Iran's nuclear agency sources told NHK that the process has started at a nuclear facility in Fordow, central Iran.
Experts say uranium enriched to 20-percent purity can be quickly converted to weapons-grade uranium.
The International Atomic Energy Agency says it has confirmed that Iran has begun the process toward 20 percent enrichment.
The latest move is seen as an attempt by the Iranian government to ramp up pressure on US President-elect Joe Biden to quickly lift sanctions on Iran.
Biden has said he will consider rejoining the nuclear deal, which US President Donald Trump unilaterally withdrew from in 2018.