Iran will not give in to US "bullying" as Washington attempts to undermine Tehran's nuclear deal with world powers, the Islamic Republic's Supreme Leader Ayatollah Ali Khamenei said.

"Iran, which is a powerful nation, will not give in to pressure and will not bow," Khamenei said in an address to police officers in Tehran on Sunday.

"The corrupt, lying, deceitful US officials insolently accuse the nation of Iran ... of lying, whereas the nation of Iran has acted honestly and will continue on this path until the end in an honest manner," said Khamenei.

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President Hassan Rouhani left on Sunday for the UN General Assembly in New York, where he is set to hold crucial talks on the 2015 nuclear deal, which eased international sanctions in exchange for curbs to Iran's atomic programme.

US President Donald Trump has threatened to tear up the deal and his administration has been looking for grounds to declare Iran in non-compliance, despite repeated UN declarations that Tehran has stuck to its commitments.

Trump must make a decision by mid-October whether to certify that Iran is complying with the deal, known as the Joint Comprehensive Plan of Action (JCPOA). If he does not, Congress has 60 days to decide whether to reimpose sanctions waived under the deal.

Khamenei said in his speech that "US bullying will not work on the Islamic Republic."

"You are the liars. The nation of Iran is standing firm and any wrong move ... will face a reaction by the Islamic Republic," said the supreme leader.

Iran's warning

Iran said last month it could abandon the nuclear agreement "within hours" if the US imposes any new penalties, after Washington ordered unilateral sanctions over Tehran's ballistic missile tests.

Rouhani, speaking on Sunday before leaving for New York, said the US should join the countries that continue to support the nuclear deal.

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The US imposed unilateral sanctions in July, saying Tehran's ballistic missile tests violated a UN resolution that endorsed the nuclear deal, and called upon Tehran not to undertake activities related to ballistic missiles capable of delivering nuclear weapons.

It stopped short of explicitly barring such activity.

Iran denies its missile development breaches the resolution, saying its missiles are not designed to carry nuclear weapons.