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By Andrea Mitchell

VIENNA, Austria — A deal with Iran described as "transformational" was coming together overnight Monday, senior Western officials told NBC News. It was expected to be announced early Tuesday.

State Department Spokeswoman Marie Harf confirmed in a tweet Tuesday that the so-called P5 + 1 members and Iran would hold a final plenary session at 10:30 a.m. local time (4:30 a.m. ET) followed by a news conference.

Citing a senior diplomat, The Associated Press reported early Tuesday that the final obstacles had been cleared and that a deal had been reached. NBC News could not immediately independently confirm that report.

Late Monday, drafts of an agreement were already being studied in Washington and other capitals, the officials said on the 17th day of negotiations toward a deal that could lift crippling sanctions against Tehran in exchange for curbs on its nuclear program.

Secretary of State John Kerry, who returned to the Coburg Palace for more talks with European Union leaders, was described as upbeat.

Among the remaining sticking points were whether to lift a U.N. arms embargo — and if so, when — and how much Tehran should have to account for past suspected nuclear activity.

There was also a last-minute dispute over how much advanced nuclear research Iran would be able to conduct at the end of deal, at least a decade from now.

Any deal would face skepticism from Congress and would likely need a strong sales campaign from the White House.

"The parts of the deal are there," a senior official from one of the six powers in talks with Iran told Reuters. "We still need to put the finishing touches together‎. All sides have to decide now. It's time to say, 'Yes.'"

During the White House briefing, press secretary Josh Earnest told reporters that the interim deal will remain in place should a deal not be reached Monday.

Earnest said the team in Vienna had made "genuine progress," especially in the last week, and that the negotiators will remain as long as the talks remain useful.

The Associated Press and Reuters contributed.