President Trump began reinstating sanctions on Iran in May. Photo: AFP
The Trump administration is to reinstate all US sanctions on Iran removed under the 2015 nuclear deal.
The White House said it was "the toughest sanctions regime ever imposed" on Tehran.
However, temporary waivers will be granted to eight countries to allow them to continue importing Iranian oil.
EU states that backed the nuclear deal have said they will protect EU firms doing "legitimate" business with Iran.
President Donald Trump withdrew from the agreement in May, describing it as "defective at its core" because it had not stopped Iran developing a ballistic missile programme and intervening in neighbouring countries.
"Sanctions are coming," he tweeted after Friday's announcement, referencing the TV series Game of Thrones and its motto "Winter is coming".
The US has been gradually re-imposing sanctions, but analysts say this move is the most important because it targets the core sectors of Iran's economy.
The agreement saw Iran limit its controversial nuclear activities in exchange for sanctions relief.
Barack Obama, the US president at the time, had argued the deal would prevent Iran from developing nuclear arms.
The UK, France, Germany, Russia and China were also parties to the 2015 accord and have stuck to it, saying Iran is complying.