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 re-impose 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. "If we achieve the deal's goals in cooperation with other members of the deal, it will remain in place," he said.