Iran and six world powers are holding fresh talks in Vienna on a final accord aimed at ending doubts over Tehran's nuclear program.
EU foreign-policy chief Catherine Ashton's spokesman, Michael Mann, said the two sides were holding "very detailed and substantial negotiations."
He added that negotiators were trying to make progress "as quickly as possible" to reach a "quality" agreement.
Iranian Foreign Minister Mohammad Javad Zarif expressed hope before the talks that enough progress could be made over the next two days to allow negotiators to start drafting a final accord by mid-May.
A U.S. official gave a similar timetable last week, voicing hope that the drafting of an agreement could begin in May.
Russia's chief negotiator said Moscow had no special expectations for this week's meeting.
Deputy Foreign Minister Sergei Ryabkov said talks on a number of issues were still in early stages and the meeting should produce a basis for further discussions.
Officials say two rounds of discussions so far have focused on what issues should be included in any long-term deal.
Iran and the six world powers -- the United States, France, Russia, China, Britain, and Germany -- reached a interim deal in November.
Under that accord, Iran agreed to scale back its nuclear program in exchange for some relief from international sanctions.
The six-month deal took effect on January 20 and was designed to buy time for talks on a final accord.
Iran has said it had useful expert-level nuclear talks with world powers in Vienna last week, addressing all major technical issues in the way of a final settlement.
Iranian negotiator Hamid Baeedinejad told the official IRNA news agency on April 6 that the results of those technical discussions would be submitted on April 7 to Zarif and Ashton.
Substantial differences remain, with the scope of Iran's uranium enrichment program as the toughest issue.
Iran argues it needs enrichment capacities to make low-enriched reactor fuel.
The United States, Britain, France, and Germany want significant cuts to limit any potential effort to turn the program into making high-enriched material for nuclear arms. Russia and China are somewhere in the middle.
With reporting by AP and Reuters