Iran's nuclear program is the focus of two separate meetings Wednesday, but no major breakthroughs are expected ahead of the country's June presidential election.
The U.N. nuclear agency is holding a 10th round of talks in Vienna since the beginning of last year with Iranian officials, as its investigators continue to seek access to Iran's nuclear facilities and documents.
The International Atomic Energy Agency's top nuclear inspector, Herman Nackaerts, told reporters before Wednesday talks that "differences remain but the agency is committed to dialog."
The IAEA is concerned about Iran's potential development of nuclear weapons, and wants to visit facilities such as the Parchin military site to resolve questions about the program.
Iran insists Parchin is a standard military facility and that its nuclear program has only peaceful purposes.
Meanwhile, in Istanbul, Iran's top nuclear negotiator Saeed Jalili will hold talks with European Union policy chief Catherine Ashton.
Their meeting follows a failed round of negotiations in early April with a group of world powers that includes the five permanent members of the U.N. Security Council, plus Germany.
Those nations are pushing for Iran to stop some of its uranium enrichment activity, while Iran has demanded they recognize its right to enrich uranium.
The U.N. Security Council has passed multiple rounds of sanctions against Iran related to its nuclear program. The United States and other Western nations have enacted their own sanctions, included targeting Iran's key oil exports.
Analysts and diplomats say any movement in the decade-old standoff will probably have to wait until after Iranians vote on June 14 for a successor to President Mahmoud Ahmadinejad.