President Donald Trump has lashed out at Iran, with national security adviser John Bolton also warning there would be "hell to pay" if Tehran crossed the US.
Speeches by the president before the UN General Assembly and by Secretary of State Mike Pompeo and Bolton at a New York City hotel marked an escalation of rhetoric.
Trump on Tuesday blasted what he called Iran's "corrupt dictatorship" and accused its leaders of enriching themselves through embezzlement and raiding state coffers to spread "mayhem".
Trump also vowed to continue to isolate Iran through US sanctions that are being re-instated following his withdrawal from the 2015 nuclear deal.
He predicted the pressure from renewed sanctions would force Iran back to the table to negotiate.
"Iran's leaders sow chaos, death and destruction," the president told the UN General Assembly.
"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 to spread mayhem across the Middle East and far beyond."
In an even more fiery speech to a group opposed to the Iran deal, Bolton went further.
"If you cross us, our allies, or our partners; if you harm our citizens; if you continue to lie, cheat and deceive, yes, there will indeed be hell to pay," he said.
"The murderous regime and its supporters will face significant consequences if they do not change their behaviour. Let my message today be clear: We are watching, and we will come after you."
Pompeo also spoke at the event hosted by United Against a Nuclear Iran, being held to coincide with UN General Assembly.
In his speech, Pompeo unveiled a long list of Iranian misdeeds, from its support of Syrian president Bashar Assad and Houthi rebels in Yemen to sponsoring or plotting attempted terrorist attacks in Africa, Asia, Europe and beyond.
"It is truly an outlaw regime," he said.
Iranian president Hassan Rouhani, who also addressed the UN, launched a scathing attack on Trump and his administration for abandoning international norms and imposing sanctions that amount to "economic terrorism".
He accused the US on Tuesday of trying to overthrow his government.
"The United States' understanding of international relations is authoritarian," Rouhani said.
"In its estimation, might makes right. Its understanding of power, not of legal and legitimate authority, is reflected in bullying and imposition."
Trump and his aides, meanwhile, stepped up their attacks on the nuclear deal, which was a signature foreign policy achievement of the Obama administration.
The other parties to the deal, under which Iran agreed to curbs on its nuclear program in exchange for sanctions relief, remain in the agreement and met on Monday in New York to reaffirm their support for it.
Aside from Iran, the other participants are Britain, China, France, Germany, Russia and the European Union.
Their meeting agreed to establish a financial facility in the European Union to facilitate payments for Iranian imports and exports including oil.
Pompeo harshly criticised those countries for attempting to subvert US sanctions.