Iran says it has resumed enriching uranium to 20 percent purity, a move that significantly violates the 2015 nuclear agreement and could complicate US efforts to rejoin the deal.
Sources at Iran's nuclear agency 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 move is seen as an attempt by the Iranian government to ramp up pressure on US President-elect Joe Biden to rejoin the nuclear deal quickly and lift sanctions imposed on the country.
The US State Department spokesperson strongly condemned the move, calling it "a clear attempt to increase its campaign of nuclear extortion."
Biden has said in the past that he will consider rejoining the nuclear deal, which US President Donald Trump unilaterally withdrew from in 2018.
He has yet to respond to Iran's latest move.
The Washington Post said in an article that "the move could complicate the incoming Biden administration's plans to restart nuclear talks with Tehran."