"I call it a milestone. It is a turning point in the negotiations," Salehi told Austrian broadcaster ORF in an interview during a visit to the Austrian capital for a United Nations conference.

"We are heading for goals that will be satisfactory for both sides. I am very optimistic and hopeful," he said, according to a German translation of remarks he made in English.

Salehi, who had said on Wednesday he was "very confident" an agreement could be reached, gave no details of the talks in Kazakhstan, but said the fact that talks would resume next month showed the process was moving forward.

Iran has struck an upbeat tone after the talks ended with an agreement to meet again, but Western officials said it had yet to take concrete steps to ease their fears about its atomic ambitions.

Rapid progress was unlikely with Iran's presidential election, due in June, raising domestic political tensions, diplomats and analysts had said ahead of the Feb. 26-27 meeting in the Kazakh city of Almaty, the first in eight months.

The United States, China, France, Russia, Britain and Germany offered modest sanctions relief in return for Iran curbing its most sensitive nuclear work but made clear that they expected no immediate breakthrough.

In an attempt to make their proposals more palatable to Iran, the six powers appeared to have softened previous demands somewhat, for example regarding their requirement that the Islamic state ship out its stockpile of higher-grade uranium.

US Secretary of State John Kerry has called the talks "useful" and said that a serious engagement by Iran could lead to a comprehensive deal in a decade-old dispute that has threatened to trigger a new Middle East war.