A final deal to restrict Iran's nuclear activity remains elusive as negotiations continue in Vienna. (Reuters)

Negotiators are on the threshold of finalizing a historic nuclear deal with Iran, diplomats said Sunday as they tried to reach consensus on a few outstanding issues that have caused the talks to drag on past one deadline after another.

Diplomats from several of the six countries negotiating with Iran said they could announce a deal as early as Monday. They cautioned that last-minute obstacles could get in the way, though virtually everyone involved has been showing signs of fatigue.

But political figures in the Middle East, Europe and Washington appeared to be preparing for an agreement that would restrict Iran’s nuclear program for more than a decade, lift international sanctions that have battered its economy and potentially alter the way Iran interacts with the rest of the world.

In the most telling indication that a comprehensive agreement was at hand, Russian Foreign Minister Sergei Lavrov arrived late Sunday. He joined U.S. Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif, who have been mainstays of the negotiations. The senior diplomats from France, Germany and Britain have been here off and on for a week. Chinese Foreign Minister Wang Yi was reported preparing to come, too.

Secretary of State John F. Kerry and U.S. Undersecretary for Political Affairs Wendy Sherman, center, are in Vienna for the high-level meetings. (Carlos Barria/AP)

“We have come a long way,” Iranian President Hassan Rouhani said in Tehran. “We need to reach a peak, and we’re very close.”

As negotiators moved into the apparent end game of their talks, Ali Rezaian released a statement Sunday saying that the trial of his brother, Washington Post Tehran bureau chief Jason Rezaian, who has been held in Iran for nearly a year and is charged with crimes including espionage, is scheduled to resume Monday. It will be the third session before Iran’s Revolutionary Court since he was detained.

Meanwhile, as diplomats gathered in the Austrian capital, where the latest round of talks kicked off June 26, some final disagreements stood in the way of a 100-page draft agreement being sent to the capitals for approval.

Zarif, taking a break at sunset on the third-floor balcony of the Coburg Palace, the hotel where the talks were conducted, said more work was needed, and he held up a sheaf of white papers that he was reading through.

The State Department tamped down speculation that a deal was ready Sunday.

“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,” said a senior State Department official, speaking anonymously under ground rules for talking to reporters.

There were other, smaller tells that most of the dickering is at the negotiators’ backs.

Kerry, who on Saturday attended back-to-back meetings that stretched over 15 hours and lasted past midnight, left his hotel late Sunday morning to attend Mass at St. Stephen’s Cathedral. And afterward, instead of rushing to meetings, he strolled through the historic center of Vienna like a typical tourist, stopping to visit a house where Mozart once lived.

“I think we’re getting to some real decisions,” Kerry said before entering his limousine. “So I will say, because we have a few tough things to do, I remain hopeful. Hopeful.”

French Foreign Minister Laurent Fabius said an end to a year and a half of intense negotiations was at hand.

“I hope that we are finally entering the last phase of this negotiation,” he told reporters outside the Coburg Palace.

Some of the delay is bureaucratic — reading through a draft, getting it approved at home and coordinating rollout announcements.

“Even if an agreement is finalized, it will take hours to check and clean up all the paperwork,” an Iranian official said Sunday afternoon. “It only requires political will at this point.”

Sunday marked the 16th straight day of high-level talks that began with the expectation that an agreement could be reached by the deadline of June 30, or shortly after. The talks have been extended three times since then, most recently until midnight Monday.

Reports from Tehran said business had virtually ground to a halt as people waited for word of the outcome. The city’s police chief announced that his force was ready to provide security for celebrating Iranians. And the speaker of Iran’s parliament, Ali Larijani, said that a comprehensive agreement deal was likely.

Some of the anticipation was more wary than gleeful.

In Israel, Prime Minister Benjamin Netanyahu criticized what he called a “parade of concessions” to Iran that he contrasted to vitriolic statements against the United States and Israel emanating from Tehran. On Saturday, for example, Iran’s supreme leader, Ayatollah Ali Khamenei, called the United States the “embodiment of global arrogance” — even though Khamenei has given his blessing to the negotiations.

“Iran does not hide its intention to continue its murderous aggression even against those with whom it is negotiating,” Netanyahu said in his weekly cabinet meeting. “Perhaps there is someone among the great powers who is prepared to capitulate to this reality that Iran is dictating, which includes its unending calls for the destruction of Israel. We will not pay the price for this.”

On CBS’s “Face the Nation” Sunday, Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, expressed doubts about Kerry’s negotiating skills with Iran and said, “I’m concerned about where we’re going.”

“You know, likely, Iran will cheat by inches, meaning they will just cheat, cheat, cheat,” he said. “And over time, it’s like boiling an egg. They end up with a nuclear weapon.”

Corker was instrumental in compromise legislation under which Congress will get 60 days to review any deal and ultimately vote to approve or reject it. It would need 67 votes to withstand a presidential veto, however.

Critics of the deal could marshal support if the final text of an agreement shows the United States and its negotiating partners had agreed to lift a U.N. embargo on conventional weapons trade — as Iran has insisted and Russia has publicly supported. The United States opposes it in part because Washington is concerned that the arms could end up helping Iran expand its regional influence by backing Syrian President Bashar Assad and groups such as the Houthi rebels in Yemen, Hezbollah in Lebanon and Hamas in the Gaza Strip.

Sen. Benjamin L. Cardin (Md), the ranking Democrat on the Foreign Relations Committee, who worked with Corker on the review legislation, seemed to suggest that Democrats may turn a wary eye on the deal.

“There is no trust when it comes to Iran,” he said in a statement. “In our deliberations we need to ensure the negotiations resulted in a comprehensive, long-lasting and verifiable outcome that also provides for snap-back of sanctions should Iran deviate from its commitments.”

Ruth Eglash in Jerusalem contributed to this report.