The Islamic republic's top diplomats have been negotiating with Washington for around two years -- first in secret, and openly since autumn 2013 -- to end a dispute over its controversial nuclear programme.
"The United States arrogantly says that if Iran makes concessions in the nuclear case, they will not at one stroke lift sanctions. With this reality, how can we trust such an enemy?" Khamenei asked in a speech in Tehran.
"We are not against negotiations... Let them talk all they want, but they must negotiate based on reality, not on imaginary points," he said.
Under an interim deal between world powers and Tehran in force since January 2014, Iran agreed to limit its uranium enrichment, a process that makes fuel for nuclear power but at high purities can produce fissile material for an atomic bomb.
In return, Iran, which denies seeking to develop an atomic weapon and insists its nuclear programme is for peaceful energy purposes only, received limited sanctions relief.
But two deadlines for a comprehensive accord with the P5+1 group of nations -- the United States, Britain, China, France, Russia plus Germany -- have since been missed.
Khamenei will have the last word on Iran's conditions for any final agreement, now facing a June 30 deadline.
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Throughout the nuclear talks he has supported Iran's diplomats while simultaneously using public speeches to express doubt about what, if anything, can be achieved.
His latest comments came one day after Foreign Minister Mohammad Javad Zarif, Iran's top negotiator, was criticised by hardline lawmakers over his handling of the talks, although he was later backed in a snap vote.
Zarif countered by saying the negotiations had changed Iran's image abroad, reversing the view that Tehran was "threatening and dangerous for world security".
"Today the world is watching Iran as a powerful and logical actor that cannot be set aside or ignored," Zarif said.
On Wednesday, however, Khamenei again raised the possibility of no final deal, calling for self-reliance to withstand crippling sanctions which have decimated Iran's oil exports, a crucial revenue earner.
"We must act so that even if the enemy does not lift the sanctions, there is no damage to the progress and well-being of the population," he said, stressing the need "to cut reliance on oil revenues".
"We must immunise ourselves against sanctions," he added.
The P5+1 talks are to resume in Geneva on January 18, following several days of bilateral meetings between Iran and member states.