US President Donald Trump will accuse Iran of failing to comply with an international nuclear deal, according to Secretary of State Rex Tillerson.
Mr Trump will refuse to certify the deal and refer it to Congress, he told reporters.
Such a move would not mean a US withdrawal from the deal that freezes Iran’s nuclear programme, but it is part of a tougher strategy on Iran.
President Trump was due to speak from the White House at 16:45 GMT.
He is under pressure at home and abroad not to scrap the seven-country deal agreed in 2015, under which Iran has halted its nuclear programme in return for the partial lifting of sanctions.
In his speech, Mr Trump is expected to accuse Tehran of pursuing “death and destruction”.
It is thought he will also focus on its non-nuclear activities, particularly those of the Revolutionary Guards (RIG), which has been accused of supporting terrorism.
What exactly is Mr Trump proposing?
During the presidential election campaign in 2016, Mr Trump pledged to throw out the agreement concluded under his predecessor, Barack Obama.
But senior cabinet officials advised him not to as Iran was abiding by its terms, so Mr Trump has opted for a middle road, reports the BBC’s Barbara Plett Usher in Washington.