Iran and world powers agreed Thursday a timetable and framework for the ambitious and arduous process of hammering out a lasting nuclear accord by July 20 that satisfies all sides.

Such a deal, if reached, should resolve the decade-old standoff over Iran's nuclear drive, silence talk of war for good and help normalize Tehran's strained international relations.

After chairing "very productive" days of talks in Vienna, EU foreign policy chief Catherine Ashton said experts would meet in early March before political directors return to Vienna on March 17.

"There is a lot to do, it won't be easy but we have made a good start," Ashton told reporters, saying negotiators had "identified all the issues we need to address."

Iranian Foreign Minister Mohammad Javad Zarif wrote on his Facebook page that the parties "also agreed to hold several meetings on a monthly basis until late May."

He told Iranian media that the atmosphere in the Austrian capital was "very serious" and "even a little bit more positive than anyone predicted," but however there is a "difficult way ahead of us."

A senior US administration official said the discussions were "very workmanlike" and "substantive, covering all the issues that need to be on the table ... We are long past speeches of ideology."

"We have begun to see some areas of agreement, as well as areas where we will have to work through very difficult issues," the official said, adding there was "extraordinary" unity among the six powers.

Diplomats said they aimed to nail down the final accord by the time a six-month freeze of certain activities agreed in Geneva expires on July 20, although this period could be extended.

Meanwhile, a senior Iranian official Thursday said Iran and Britain officially resumed diplomatic relations severed by London in 2011.

"From today relations between Iran and Britain are resumed at the non-resident charges d'affaires level," Deputy Foreign Minister Majid Takht-Ravanchi told ISNA, adding that the countries' flags were raised atop their embassies in Tehran and London.

The British Foreign Office confirmed the news.

Britain had ordered the closure of Iran's Embassy in London after shuttering its own in Tehran when hundreds of Islamist students stormed the embassy as well as the ambassador's residence in November 2011.

The students protested against Western sanctions over Tehran's disputed nuclear program.

AFP