(Adds Press TV on deal unlikely Thursday)

By Louis Charbonneau and Parisa Hafezi

VIENNA, July 9 (Reuters) - 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.

Russian Deputy Foreign Minister Sergei Ryabkov said on Thursday he could not rule out that there would be an agreement in the coming hours.

But the Iranian state broadcaster Press TV cited an Iranian official as saying it was unlikely an agreement would be reached on Thursday.

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."

However a senior Western diplomat said it was "very doubtful" the talks would finish on Thursday.

Western countries accuse Iran of seeking the capability to build nuclear weapons, while Tehran says its programme is peaceful. A deal would depend on Iran accepting curbs on its nuclear programme in return for the easing of economic sanctions imposed by the United Nations, United States and European Union.

A successful deal could be the biggest milestone in decades towards easing hostility between Iran and the United States, enemies 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.

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 Thursday, the review period would be doubled 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.

"96 PERCENT COMPLETE"

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.

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 a U.N. Security Council arms embargo was among the most contentious unresolved points.

Tehran says the U.N. embargo on conventional weapons must be lifted in a nuclear deal. Western countries are keen not to allow Iran to begin importing arms because of its role supporting sides in conflicts in the Middle East.

Iran has powerful support on this issue from Russia. Foreign Minister Sergei Lavrov said the U.N. arms embargo should be among the first sanctions lifted in a deal.

"We are calling for lifting the embargo as soon as possible and we will support the choices that Iran's negotiators make," he said at a summit of BRICS countries - Brazil, China, India, Russia and South Africa.

Iran's President Hasan Rouhani was also at the summit and due to meet Russian President Vladimir Putin.

Before leaving 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."

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 U.N. weapons and missile sanctions were imposed with Russia's consent nearly a decade ago, but Moscow has become hostile to the idea of sanctions since the United States and European Union began sanctioning it for annexing Ukraine's Crimea peninsula last year. Moscow and Tehran are interested in finishing a deal on the sale to Iran of Russian anti-aircraft missiles.

The Kremlin signed a decree in April lifting a self-imposed ban on the delivery of the S-300 missile system to Iran, though the missiles have yet to be delivered. (Additional reporting by John Irish and Arshad Mohammed and Shadia Nasralla in Vienna and Katya Golubkova and Denis Pinchuk in Russia, writing by Louis Charbonneau; Editing by Anna Willard and Peter Graff)