US President Donald Trump says he will withdraw the US from an Obama-era nuclear agreement with Iran.
Calling it "decaying and rotten", he said the deal was "an embarrassment" to him "as a citizen".
Going against advice from European allies, he said he would reimpose economic sanctions that were waived when the deal was signed in 2015.
In response, Iran said it was preparing to restart uranium enrichment, key for making both nuclear energy and weapons.
Iran's President Hassan Rouhani said: "The US has announced that it doesn't respect its commitments.
"I have ordered the Atomic Energy Organisation of Iran to be ready for action if needed, so that if necessary we can resume our enrichment on an industrial level without any limitations."
He said he would "wait a few weeks" to speak to allies and the other signatories to the nuclear deal first.
Mr Trump had previously complained that the deal only limited Iran's nuclear activities for a fixed period; had failed to stop the development of ballistic missiles; and had handed Iran a $100bn (£74bn) windfall that it used "as a slush fund for weapons, terror, and oppression" across the Middle East.
"It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of this deal," Mr Trump said.
"The Iran deal is defective to its core."
Former President Barack Obama, who signed the deal on behalf of the US three years ago, called Mr Trump's announcement "misguided".
When will the sanctions restart?
The US Treasury said economic sanctions would not be reimposed on Iran immediately, but would be subject to 90-day and 180-day wind-down periods.
In a statement on its website, it said sanctions would be reimposed on the industries mentioned in the 2015 deal, including Iran's oil sector, aircraft exports, precious metals trade, and Iranian government attempts to buy US dollar banknotes.
