Iran's deputy negotiator Ali Bagheri on Sunday blamed the slow progress of the Baghdad talks on the P5+1's "lack of preparedness," Iran's official news agency IRNA reported.
In a letter to the European Union's Helga Schmid, Bagheri said that Iran had provided proposals for the framework of talks as well as issues to be discussed between the two sides.
The letter came in response to a message that Schmid send Bagheri on Thursday, in which she said that Iran should start detailed talks with world powers to address its production of high-grade uranium.
Schmid said an offer by the six powers negotiating with Iran to resolve a standoff over its nuclear work, put forth at a round of talks in Baghdad last month, was still on the table.
"It remains the collective conviction of the (six powers) that this proposal continues to represent the most promising basis for moving our discussions towards achieving concrete results at an early stage," she wrote in a letter to Bagheri.
Reuters contributed to this report