The envoys said a provisional agreement may be reached even earlier by today. 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.
Senior US and Iranian officials suggested, however, there was not enough time to reach a provisional deal by the end of the day.
"We are working hard, but a deal tonight is simply logistically impossible," the Iranian official told AP, 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, saying "major issues remain to be resolved."
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Despite the caution, the negotiators appeared to be on the cusp of an agreement.
US Secretary of State John Kerry, who on Thursday had threatened to walk away from the negotiations, said today that "a few tough things" remain in the way but added "we're getting to some real decisions."
French Foreign Minister Laurent Fabius also was cautiously optimistic, telling reporters today, "I hope that we are finally entering the last phase of this negotiation."
Movement toward a deal has been marked by years of tough negotiations. The pact is meant to impose long-term, verifiable limits on nuclear programs that Tehran could modify to produce weapons. Iran, in return, would get tens of billions of dollars in sanctions relief.
As the weekend approached, Kerry declared the talks couldn't go on indefinitely and warned that the US could walk away from the negotiations.