US National Security Adviser John Bolton has warned Iran’s rulers that there will be “hell to pay” if they harm the US, its citizens or allies.
His comments came hours after President Donald Trump accused Iran of sowing “chaos, death and destruction” across the Middle East.
Iranian President Hassan Rouhani responded by criticising the Trump administration for its hostility.
The US recently imposed sanctions after leaving the 2015 Iran nuclear deal.
The accord, negotiated by former President Barack Obama, saw Iran limit its nuclear activities in exchange for sanctions relief.
Mr. Trump believes renewed economic pressure will force Iran to agree to a new deal.
Mr. Bolton said the “murderous regime” of “mullahs in Tehran” would face significant consequences if they continued to “lie, cheat and deceive”.
The former US envoy to the UN was speaking at an anti-Iran conference in New York on Tuesday.
“If you cross us, our allies, or our partners; if you harm our citizens,” he said, “there will indeed be hell to pay.”
“Let my message today be clear: We are watching, and we will come after you.”
Mr. Bolton, who has argued for military action against Iran in the past, said the US would be aggressive in enforcing economic sanctions.
Meanwhile, President Trump defended his decision to abandon the nuclear deal while speaking at the UN General Assembly in New York.
“Iran’s leaders sow chaos, death and destruction,” he said.
“They do not respect their neighbours or borders or the sovereign rights of nations. Instead, Iran’s leaders plunder the nation’s resources to enrich themselves and spread mayhem across the Middle East and far beyond.”
Mr. Rouhani said dialogue should begin by ending threats and what he called “unjust sanctions”, adding that no nation could be brought to the negotiating table by force.
“The United States’ understanding of international relations is authoritarian. In its estimation might makes right. Its understanding of power, not of legal and legitimate authority, is reflected in bullying and imposition,” he said in New York.
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Iran has accused Mr. Trump of waging “psychological warfare” against it and denies any involvement in terrorism. It insists its nuclear programme is entirely peaceful.
Padma Lakshmi: TV host says why she kept silent on her rape
US television host Padma Lakshmi has explained why she kept silent after being raped at the age of 16.
In a piece for the New York Times, Lakshmi, 48, says she was raped on New Year’s Eve by her boyfriend.
But she said she began to feel the attack was her fault, and that she understands why women might not disclose sexual assaults.
Her article was prompted by allegations surrounding Donald Trump’s choice for the Supreme Court.
The US president has cast doubt on two women who have accused Judge Brett Kavanaugh of sexual misconduct.
The allegations against Judge Kavanaugh, which he has repeatedly denied, date back to the 1980s.
Ms. Lakshmi wrote: “President Trump tweeted that if what Dr Ford said was true, she would have filed a police report years ago.”
“But I understand why both women would keep this information to themselves for so many years without involving the police. For years, I did the same thing.”
The Top Chef host says she met her boyfriend, who was then 23, while working in Los Angeles.
After attending a couple of New Year’s Eve parties, she said they went to his apartment where she fell asleep, and woke up with him on top of her.
“I asked, ‘What are you doing?’ He said, ‘It will only hurt for a while.’ ‘Please don’t do this,’ I screamed.”
He later drove her home, and Ms. Lakshmi said she was in shock.
“I didn’t report it. Not to my mother, not to my friends and certainly not to the police.”