"Negotiations with the USA would definitely harm us and they are forbidden", Khamenei said, adding that the Americans had proven they could not be trusted.
"Not that sanctions don't have an impact, but the main factor is how we handle them", he added.
The sanctions target Iran's trade in gold and other precious metals, its purchases of U.S. dollars and its auto industry.
Thousands of Iranians have held protests in recent months blaming the government's expansionist foreign policy for the country's economic crisis, citing the government's costly war in Syria and Yemen as well as the ongoing confrontation with the U.S.
Defying US demands to curb its missile program, Iran unveiled a new generation of its "Fateh Mobin" short-range ballistic missile on Monday, state TV reported.More news: Liverpool vs. West Ham live stream
Khamenei, whose remarks on Monday come amid a sharp fall in the rial currency that has prompted angry protests, criticized the government of President Hassan Rouhani, a pragmatist cleric who championed the 2015 deal aimed at ending Iran's political and economic isolation.
Khamenei referred specifically to the collapse in the currency, which has seen half its value wiped since early April.
The conservative establishment appears determined to show it is responding firmly to public anger, particularly over corruption.
State media said that 67 officials had been sacked and arrested on suspicion of crimes ranging from corruption to economic sabotage.
Khamenei approved a written request from the head of the judiciary, Sadegh Larijani, to set up special revolutionary courts to try people quickly for economic crimes.More news: Thomas Markle reportedly fears he won’t see royal daughter Meghan again
With the nuclear deal crumbling, Rouhani finds himself with little to show for his five years in power and increasingly under fire from all sides.
The supreme leader added that if Tehran acted "better and stronger" itself USA sanctions would have not been so harmful.
"Today's livelihood problems do not emerge from outside, they are internal", Ayatollah Khamenei said in another tweet.
Unless Iran's clerical rulers comply with the United States demands, more sanctions targeting Iran's oil and shipping industries are set for November.More news: Omarosa Secretly Recorded John Kelly Firing Her