Updated at 4:20 p.m. ET

The Associated Press reports that negotiators are prepared to announce a historic nuclear deal with Iran on Monday, but the U.S. State Department and some Iranian diplomats are tamping down speculation.

The AP says: "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 and a formal agreement must still be reviewed by leaders in the capitals of Iran and the six world powers at the talks."

But a senior State Department official, who asked not to be named, said: "We have never speculated about the timing of anything during these negotiations, and we're certainly not going to start now — especially given the fact that major issues remain to be resolved in these talks."

NPR's Peter Kenyon, reporting from the talks in Vienna, says Russian Foreign Minister Sergei Lavrov was returning to the talks, adding to speculation that the sides are close to an agreement. British Foreign Secretary Philip Hammond is also expected to arrive back in Vienna on Monday.

Publicly, Secretary of State John Kerry, who on Thursday 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," according to the AP.

Later, Iran's foreign minister, Mohammad Javad Zarif, told reporters that negotiators will be working Monday but did not elaborate.

Tehran's top negotiator, Ali Akbar Salehi, was quoted by the Iranian Students' News Agency, or ISNA, as saying: "Technical discussions are almost over and the text regarding the technical issues with their annexes is almost finished."

However, another Iranian official in Vienna said it was "too optimistic" to predict a breakthrough.

"Even if an agreement is finalized, it will take hours to check and clean up all the paperwork," the official was quoted by The Washington Post as saying. "It only requires political will at this point."

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