TEHRAN: Iran's supreme leader Ayatollah Ali Khamenei lashed out at the US yesterday, saying Tehran will reject any talks backed by Washington because it cannot 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," he said.

"Iran will not be fooled by the superficial conciliatory tone of the US," 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.

"(Obama) repeatedly sent oral and written messages to come and change the page... We said we will not pre-judge. We will see their action," Khamenei said.