TEHRAN - Iran's supreme leader Ayatollah Ali Khamenei lashed out at the United States on Tuesday, saying Tehran will reject any talks backed by its arch-foe because Washington is not to be trusted.

Khamenei's salvo raised the possibility that a Washington-backed nuclear fuel deal for a Tehran research reactor may be derailed, despite world powers turning up the heat on Iran to accept the UN-brokered offer.

Dismissing US President Barack Obama's series of diplomatic overtures towards Iran, the all-powerful Iranian leader said Washington wanted to negotiate with Tehran but its talks were full of "threats."

"Every time they have a smile on their face, they are hiding a dagger behind their back," said the country's top cleric who has the final say on all Iranian national issues.

"Iran will not be fooled by the superficial conciliatory tone of the United States," he said in a speech to students on the eve of the 30th anniversary of the seizure of US embassy in Tehran by Islamist students.

"This new American president repeatedly sent us oral and written messages to come and change the page... to come and cooperate in solving the problems of the world. We said we will not pre-judge. We will see their action and see what they do about the change," Khamenei said.

"But in the past eight months what we have seen is contradictory to what they say. They are telling us to negotiate, but alongside the negotiation there is a threat that if the negotiation does not bear the desired results, then we will do this and we will do that.