"We've put some proposals forward, which will hopefully help Iran show some flexibility," said Michael Mann, a spokesman for Catherine Ashton, the European Union foreign policy chief who is chairing the talks.

"What comes out of this meeting does depend very much on the Iranian reaction to what we propose," Mann added. "The onus is very much on the Iranians. They are of course in breach of U.N. security council resolutions and board of governance IAEA resolutions as well."

In a separate briefing, a U.S. official told journalists, on condition of anonymity, that the new package would include "some additional sanctions relief."

In his speech on February 16th, Iran's supreme leader argued that the P5 plus 1 countries, and in particular the U.S., were insincere when they talked about working toward a negotiated solution.

"This kind of negotiation does not serve any purpose. It will not reach any results," Khamenei said.

"They are not after genuine and fair negotiations. Rather, that they are pursuing the goal of making the Iranian nation surrender."

Analysts have suggested it may be difficult for the Iranian delegation to agree to any substantial compromise since Iran is gearing up to elect a new president in June.

Asked about this theory, the U.S. official told journalists elections "may constrain their political space."

But, the official added, "when it comes to the nuclear file, the decisions rest in the hand of the supreme leader and he is not going to change in June."