Iran’s foreign minister said Tehran and six world powers have made "significant progress" at marathon all-night nuclear talk sessions meant to reach a preliminary understanding on the Iranian nuclear talks

Iran's foreign minister said Tehran and six world powers have made "significant progress" at marathon all-night nuclear talk sessions meant to reach a preliminary understanding on the Iranian nuclear talks.

But Mohammad Javad Zarif said Thursday that an agreement still remains to be written, adding there is not yet a "final result."

Asked if that would be possible during the day, Iranian nuclear chief Ali Akbar Salehi said "the sunrise is just ahead."

"We are studying the technical issues and we are close to the sunrise," Salehi told reporters after days of marathon talks with negotiators from the six world powers in Lausanne, Switzerland, on Thursday. 

He pointed to the trend of the nuclear negotiations, and said, "Thanks God, we are proceeding well." "We have reached a common understanding on a considerable section of the issues," Salehi said after his meeting with US Secretary of Energy.

With even a vague outline of an Iran nuclear deal eluding their grasp, negotiators went into double overtime Wednesday night and Thursday morning in a marathon attempt to find common ground forge a final deal by the end of June.

At around 6 am local time Thursday, State Department spokeswoman Marie Harf tweeted that the talks had broken after an all-night session and would resume in a few hours. "We continue to make progress but have not reached a political understanding," Harf said in announcing Kerry's decision.

German Foreign Minister Frank-Walter Steinmeier said negotiators were still facing a "tough struggle."