Iran and six world powers were close to an historic nuclear agreement on Thursday that could resolve a more than 12-year dispute over Tehran's nuclear ambitions, but they remained deadlocked on the issue of Iranian arms and missile trade.
Over the past two weeks, Iran, the United States, Britain, France, Germany, Russia and China have twice extended a deadline for completing a long-term deal under which Tehran would curb sensitive nuclear activities for more than a decade in exchange for sanctions relief.
U.S. Energy Secretary Ernest Moniz and Iranian nuclear chief Ali Akbar Salehi were meeting on Thursday morning.
Salehi told reporters, "Hopefully today is the last day." Moniz added: "We're going to resolve the last issues, if we can."
The latest extension of the talks to Friday left open the possibility an agreement would not arrive in time to secure a 30-day review period by the Republican-dominated U.S. Congress.
If a deal is sent to Congress after July 9 - Thursday - the period grows to 60 days, increasing the chance that the deal could unravel.
U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have been meeting daily for two weeks to overcome the last remaining obstacles to a deal. French Foreign Minister Laurent Fabius and his British and German counterparts have also rejoined the negotiations.
The White House said President Barack Obama and his national security team held a video conference on Wednesday with Kerry, Moniz and the U.S. negotiating team in Vienna.
"The President reviewed the progress of negotiations to date, and provided guidance related to our ongoing efforts to achieve a good deal between the P5+1 and Iran that meets our requirements," the White House said in a statement.
Iran's Deputy Foreign Minister Abbas Araqchi said the main text of the agreement, as well as five technical annexes, were "around 96 percent complete." While the lifting of sanctions was largely agreed, Araqchi said Tehran's demand for an end to the U.N. Security Council arms embargo was among the most contentious unresolved points.
Tehran has powerful support on this issue from Russia, one of the six nations negotiating with Iran - Britain, China, France, Germany, Russia and the United States.
Speaking at a summit of the BRICS countries - Brazil, Russia, India, China and South Africa - in Ufa, Russia, Russian Foreign Minister Sergei Lavrov said the United Nations arms embargo should be among the first sanctions lifted if there is a nuclear deal.
"We are calling for lifting the embargo as soon as possible and we will support the choices that Iran's negotiators make," Lavrov said.
Earlier this week, a senior Western diplomat said that despite Russia's and China's known opposition to the arms embargo and sanctions on Iran's ballistic missile programme, they had decided not to break ranks with the West on the issue.
The United States and its European allies are determined to keep the arms and missile sanctions in place in the event of a nuclear deal with Iran. Tehran says the missile and arms restrictions are unrelated to the nuclear talks and should therefore be terminated if a nuclear agreement is struck.
Russia has become increasingly allergic to the idea of sanctions on any nation since the United States and European Union began sanctioning it for annexing Crimea from Ukraine. The West accuses Moscow of supporting pro-Russian rebels in eastern Ukraine, though the Kremlin denies the charge.
There may also be commercial dimensions to Russia's support for Iran on missiles and arms trade. Moscow and Tehran are interested in finishing their deal on the sale to Iran of Russia's advanced S-300 missile system.
The Kremlin signed a decree in April lifting a self-imposed ban on the delivery of the S-300s to Iran, though the missiles have yet to be delivered.
Iranian President Hassan Rouhani was also attending the BRICS summit and was expected to meet with Russian President Vladimir Putin.
Before his departure from Tehran on Wednesday, Rouhani was quoted by the semi-official Mehr news agency as saying that, "Iran is preparing itself for after the negotiations and after sanctions, in which our relations with other countries ... will expand."
Western countries accuse Iran of seeking the capability to build nuclear weapons, while Tehran says its programme is peaceful.
A successful deal could be the biggest milestone in decades towards easing hostility between Iran and the United States, foes since Iranian revolutionaries stormed the U.S. embassy in Tehran in 1979.
It would also be a political success for both U.S. President Barack Obama and Iran's pragmatic President Hassan Rouhani, both of whom face scepticism from powerful hardliners at home.