VIENNA - After more than two weeks of marathon negotiations, Iran and six world powers were close to nailing down an historic nuclear deal that would bring sanctions relief in exchange for curbs on Tehran's atomic program, diplomats said on Sunday.

But Iranian and Western officials said it was unlikely they would be able to finalize an agreement on Sunday, saying the earliest an agreement could be ready was more likely Monday.

"We are working hard, but a deal tonight is simply logistically impossible," Alireza Miryousefi, a spokesman for the Iranian delegation, said on Twitter. "This is a 100-page document, after all."

A Western official said Tehran and Washington would need time to consult their capitals once an agreement was reached.

U.S. Secretary of State John Kerry cautioned that some difficult issues remained on the 16th day of ministerial negotiations between Iran, the United States, Britain, France, Germany, Russia and China.

"I think we're getting to some real decisions," Kerry told reporters in the Austrian capital. "So I will say, because we have a few tough things to do, I remain hopeful. Hopeful."

Several diplomats said an agreement that would end more than a year and a half of negotiations was so close that it could come as early as on Sunday. In a sign that something might be in the works, both Russian Foreign Minister Sergei Lavrov and Chinese Foreign Minister Wang Yi were also due to join the talks on Sunday.(Reuters)