- "We have identified all of the issues we need to address" for final deal, says Ashton
- The next round of talks will begin on March 17, EU foreign policy chief says
- U.S. State Department spokeswoman says talks have been "constructive and useful"
- Western powers suspect Iran wants to develop a nuclear weapon, a claim it denies
Six world powers and Iran have reached a deal on the framework for comprehensive negotiations over Tehran's nuclear program, EU foreign policy chief Catherine Ashton said Thursday.
"We have identified all of the issues we need to address in reaching a comprehensive and final agreement," Ashton said, speaking alongside Iranian Foreign Minister Mohammad Javad Zarif, in Vienna, Austria.
"There is a lot to do. It won't be easy, but we've made a good start" following "three very productive days" of talks, Ashton added.
In addition to political discussions, the two sides have started technical work, Ashton said, and have set a timetable for meetings over the next four months, with a framework for further deliberations.
Technical experts will meet in early March, she said, and the next round of talks between the so-called P5+1 -- the United States, China, Russia, Britain, France and Germany -- and Iran will start March 17.
Zarif then delivered the same statement in Persian.
The latest talks began Tuesday.
U.S. State Department spokeswoman Marie Harf said Wednesday the meetings had been "constructive and useful."
The latest talks come after an interim deal was forged in November, under which Iran agreed to roll back parts of its nuclear program in return for relief from some sanctions. That agreement came into effect in January.
The challenge now is to reach a permanent deal acceptable to all sides.
The United States and its allies believe Iran is trying to develop a nuclear weapon, while Tehran has said its atomic efforts are peaceful.
Wendy Sherman, a senior State Department official and lead negotiator for the United States on the interim deal, told CNN's Wolf Blitzer from Vienna this week that any final agreement will be contingent on Iran taking "concrete" verifiable steps that prevent it from ever obtaining a nuclear weapon.
Deputy Foreign Minister Abbas Araqchi insisted Tuesday that the "halting of Iran's (nuclear) program and dismantling Iran's nuclear facilities are not on the agenda," the semi-official Mehr news agency reported.
Harf said specific issues such as dismantlement would be on the table in later discussions.
"We know both sides come to the negotiating table with certain positions in mind, clearly, but we do think that we have made some progress over these last few days and, hopefully, can continue to build on that going forward," she told reporters in Washington.