American and Iranian negotiators will resume bilateral talks on Iran's disputed nuclear program next week in Geneva as a third deadline for a deal looms, a US official said Thursday.

Acting Deputy Secretary Wendy Sherman, who has led the US delegation for more than two years, will head the team of senior officials and advisors to the next round of bilateral negotiations on January 15-17 in the Swiss city, the State Department said.

The two sides will also be joined by the EU's political director Helga Schmid who will chair the next round of talks between the group known as the P5+1 and Iran in Geneva on January 18.

In a sign that there are still tough talks ahead, Iran's supreme leader Ayatollah Ali Khamenei Wednesday voiced his distrust of Washington as he weighed the prospects of a complex nuclear deal.

Under an interim deal between world powers and Tehran in force since January 2014, Iran agreed to limit its uranium enrichment, which can produce material for an atomic bomb.

In return, Iran, which denies seeking to develop an atomic weapon insisting its nuclear program is for peaceful energy purposes only, received limited relief from crippling sanctions.

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.

The teams have now set a July 1 ultimatum, although they hope to reach a framework accord sometime in March, leaving the most complex technical details to be finalized afterwards.

Amid pressure from the US Congress to impose more sanctions, Washington has also insisted that in the initial stages of a deal it would only suspend not entirely lift the measures.

"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 said 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."

Khamenei will have the last word on Iran's conditions for any final agreement.