Iranian President Hassan Rouhani has cautioned that further measures still needed to be resolved before a nuclear deal could be reach in Vienna, ISNA news agency reported.
"It might seem we have reached the top of the mountain. But no, there are still steps needed to be taken," the agency quoted him as saying. "Even if we fail ... we have performed our duty," the Iranian president was quoted as having said on Sunday.
Rouhani's comments came as negotiations were said to be nearing an end with speculation is rife that an announcement would come on Monday, after diplomats told reporters they were on the cusp of a deal.
The envoys said a provisional agreement may be reached by Sunday night.
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.
Al Jazeera's James Bays, reporting from Vienna, said there was still a lot of paperwork to get through before a final deal could be agreed upon.
Senior US and Iranian officials had previously suggested to the Associated Press news agency that there was not enough time to reach a provisional deal by the end of Sunday.
The officials, who are at the talks, asked for anonymity because they weren't authorised to speak publicly.
"We are working hard, but a deal tonight is simply logistically impossible," the Iranian official said, noting that the agreement would run to 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."
Celebrations expects in Tehran
The current round of nuclear talks is now in its 16th day and has been extended three times since the first deadline of March 31 was missed. The mood among negotiators had turned more somber each time a new target date was set.
Diplomats familiar with the talks said most of the nuts and bolts of implementing the deal have been agreed upon. But over the past week, issues that were previously on the back burner have led to new disputes.
Among them is Iran's demand for the lifting of a UN arms embargo and its insistence that any UN Security Council resolution approving the nuclear deal be written in a way that does not describe its nuclear activities as illegal.
Iranians were said to be preparing to celebrate in the event of an agreement. Iran's semi-official ISNA news agency reported that deputy police chief Brigadier General Saeed Montazer al-Mahdi said the authorities were fully prepared for such celebrations.
Source: Al Jazeera and agencies