"With regard to our situation do not be worried at all. Nobody can do a damn thing. Be sure. There is no doubt about it," Ayatollah Khamenei said in one of his recent speeches which was published on Wednesday, only a day after the new US sanctions took effect.
US President Donald Trump said on Tuesday that the new sanctions, which were lifted under a 2015 international nuclear deal, were the most biting sanctions ever imposed. He warned that companies doing business with Iran would be barred from the United States.
Iran has denounced as "US unilateralism" the reimposition of sanctions in line with Trump’s decision to pull out of the agreement over Iran’s nuclear program.
Iran's Ambassador to the United Nations Qolamali Khoshrou, in a commentary published by Britain's Guardian newspaper on Wednesday, said that Trump was making history by violating a UN Security Council resolution it voted for three years ago.
The resolution, which underpinned the pact between Iran and six powers, calls on UN member states to refrain "from actions that undermine implementation of commitments" under the nuclear accord.
US officials have said in recent weeks that they aim to pressure countries to stop buying oil from Iran.
Iranian Foreign Minister Mohammad Javad Zarif said on Wednesday that a US plan to reduce Iran's oil exports to zero will not succeed.
"If the Americans want to keep this simplistic and impossible idea in their minds they should also know its consequences," Zarif told Iran Daily.
"They can’t think that Iran won’t export oil and others will export."
Last month, Trump offered to meet Iran's president "without preconditions". But President Hassan Rouhani on Monday said there could be no talks as long as Washington was reneging on the nuclear deal.
Rouhani, speaking in a meeting with North Korea's foreign minister on Wednesday, said that America cannot be trusted.
"Today, America is identified as an unreliable and untrustworthy country in the world which does not adhere to any of its obligations," Rouhani said.
Zarif said that Oman and Switzerland have acted as mediators in talks with America in the past but that there are currently no direct or indirect talks being held with the United States.
Reuters and ISNA contributed to this story.