US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif will face off in Vienna Sunday with seven days left to bridge major differences and strike a historic nuclear deal.

US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif will face off in Vienna Sunday with seven days left to bridge major differences and strike a historic nuclear deal.

Kerry and the foreign ministers of Britain, France and Germany will also use their presence in the Austrian capital to discuss an international push for a ceasefire in Gaza.

Iran's talks with the five permanent members of the UN Security Council plus Germany are aimed at a grand bargain reducing in scope Iran's nuclear activities in return for sanctions relief.

Officials on both sides say that the talks, which on July 3 entered their sixth and final round, have made some progress, with Iran's chief negotiator saying Saturday that a draft accord was 60-65 percent completed.

But one key notable issue remains: uranium enrichment, a process which can produce nuclear fuel, which is Iran's goal.

"We have made some progress but on some key issues, Iran has not moved from their ... unworkable and inadequate positions", a senior US official said on Saturday.