Iranian negotiator Ali Bagheri warned this month's talks in Moscow over Iran's nuclear program could stall because of faulty preparation.
Bagheri said the sides agreed to hold preliminary talks to clarify the agenda for the Moscow round, set for June 18-19.
The official IRNA news agency says Bagheri made the complaint in a letter to senior EU official Helga Schmid on Sunday.
Concerned that Iran might be aiming toward nuclear weapons, the West wants to stop Iran's 20 percent uranium enrichment program. Iran insists its nuclear program is peaceful. In exchange for discussing enrichment, Iran wants the West to ease sanctions.
Schmid has indicated there is no need for preliminary talks. The EU official said the six-power proposal at the Baghdad talks addresses "our key concerns on the 20 percent enrichment activities."
On Friday, a U.S. envoy said that lack of progress in talks between Iran and the International Atomic Energy Agency (IAEA) is disappointing and it shows Tehran's continued failure to abide by its commitment to the UN nuclear watchdog.
The IAEA, a Vienna-based UN agency, said no progress had been made in the meeting aimed at sealing a framework deal on resuming its long-stalled investigation.
Six world powers were scrutinizing the IAEA-Iran meeting to judge whether the Iranians were ready to make concessions before a resumption of wider-ranging negotiations with them in Moscow on June 18-19 on the decade-old nuclear dispute.
"We're disappointed," Robert Wood, the acting U.S. envoy to the IAEA, told Reuters in an emailed comment.
"Yesterday's outcome highlights Iran's continued failure to abide by its commitment to the IAEA, and further underscores the need for it to work with the IAEA to address international community's real concerns," he said.
The IAEA had been pressing Tehran for an accord that would give its inspectors immediate access to the Parchin military complex, where it believes explosives tests relevant for the development of nuclear arms have taken place, and suspects Iran may now be cleaning the site of any incriminating evidence.
The United States, European powers and Israel want to curb Iranian atomic activities they fear are intended to produce nuclear bombs. The Islamic Republic says its nuclear program is meant purely to produce energy for civilian uses.
Both the IAEA and Iran - which insists it will work with the UN agency to prove allegations of a nuclear weapons agenda are "forged and fabricated" - said before Friday's meeting that significant headway had been made on the procedural document.
But differences persisted over how the IAEA should conduct its inquiry, in which UN inspectors want access to sites, documents and officials.
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