However, they did acknowledge that their plan to get a deal in the coming months was very ambitious.
By late July, Western governments hope to hammer out an accord that would lay to rest their suspicions that Iran is seeking the capability to make a nuclear bomb, an aim it denies, while Tehran wants an end to economic sanctions.
Wide differences remain on how this could be achieved, although the two sides said yesterday they had agreed an agenda and timetable for such an accord during meetings in the Austrian capital this week.
EU foreign policy chief Catherine Ashton said: "We have had three very productive days, during which we have identified all of the issues we need to address in reaching a comprehensive and final agreement."
Baroness Ashton, who speaks on behalf of the six powers - the US, Russia, China, France, Britain and Germany - added: "There is a lot to do. It won't be easy but we have made a good start."
Senior diplomats from the six nations, as well as Baroness Ashton and Iran's Foreign Minister Mohammad Javad Zarif, will meet again in Vienna on March 17, and hold a series of further discussions ahead of the July deadline.
Tehran says that its nuclear programme has no military aims.