Negotiators at the Iran nuclear talks plan to announce Monday that they've reached a historic deal capping nearly a decade of diplomacy that would curb the country's atomic program in return for sanctions relief, two diplomats told The Associated Press on Sunday.

The envoys said a provisional agreement may be reached even earlier - by late Sunday. But they cautioned that final details of the pact were still being worked out. Once it is complete, a formal, final agreement would be open to review by officials in the capitals of Iran and the six world powers at the talks, they said.

Senior US and Iranian officials suggested, however, that there might not be enough time to reach a deal by the end of Sunday and that the drafting of documents could bleed into Monday.

All of the officials, who are at the talks in Vienna, Austria, demanded anonymity because they weren't authorized to discuss the negotiations publicly.

"We are working hard, but a deal tonight is simply logistically impossible," the Iranian official said, noting that the agreement will run roughly 100 pages.

The senior US official declined to speculate as to the timing of any agreement or announcement but said "major issues remain to be resolved".

Despite the caution, the negotiators appeared to be on the cusp of an agreement.

US Secretary of State John Kerry, who on July 9 had threatened to walk away from the negotiations, said Sunday that "a few tough things" remain in the way but added "we're getting to some real decisions".

En route to Mass at Vienna's gothic St. Stephens Cathedral, Kerry said twice he was "hopeful" after a "very good meeting" on July 11 with Iranian Foreign Minister Mohammad Javad Zarif, who had Muslim services on July 10. The two met again in the early evening on July 11.

French Foreign Minister Laurent Fabius also was cautiously optimistic, telling reporters Sunday: "I hope that we are finally entering the last phase of this negotiation."

In Iran, President Hassan Rouhani said an agreement was close, but not quite done, describing the negotiations as "still steps away from reaching the intended peak".

In another sign that a deal could soon be sealed, Russian news agencies reported that Russian Foreign Minister Sergey Lavrov had arrived in Vienna. Chinese Foreign Minister Wang Yi was also expected later in the day.