* Iran meets IAEA day after talks at UN in New York
* Will meet again for "substantial" talks on Oct. 28
* New president seeks end to dispute within six months
By Fredrik Dahl
VIENNA, Sept 27 (Reuters) - Iran and the U.N. nuclear agency held "constructive" talks on Friday and made plans to meet again in one month, adding to momentum for a negotiated end to a standoff that could otherwise potentially flare into war.
The discussions in Vienna, home of the International Atomic Energy Agency (IAEA), took place as new President Hassan Rouhani was telling world powers in New York he wanted a deal within months to end the long-running dispute.
The IAEA talks are distinct from Iran's meetings with world powers, but both diplomatic tracks centre on suspicions that Iran may be seeking the capability to assemble nuclear bombs behind the facade of a civilian atomic energy programme.
Israel and the United States have threatened possible pre-emptive strikes on Iran if diplomacy fails. Iran says its nuclear programme is a peaceful bid to generate electricity, and not aimed at building weapons.
Herman Nackaerts, IAEA deputy director general, said the discussions, at Iran's diplomatic mission in Vienna, had been "very constructive" but gave no details. At the next meeting on Oct. 28, Iran and the IAEA would "start substantial discussions on the way forward to resolve all outstanding issues," he said.
That would be almost two weeks after Iran meets the six world powers again, in Geneva on Oct. 15-16, as part of what European Union foreign policy chief Catherine Ashton called an "ambitious timetable" to address Western concerns.
Analysts suspect that Iran may seek to use the IAEA talks to help win relief from sanctions as part of any wider political settlement with the powers - the United States, France, Britain, Germany, Russia and China.
The IAEA - in its role of preventing the spread of atomic arms - wants a deal allowing it to resume a long-stalled inquiry into suspected nuclear weapons research in Iran.
The Vienna meeting - the 11th since January 2012 - was shorter than previous ones, just over four hours, suggesting that any concrete progress would have to wait for the follow-up. The IAEA is seeking access to Iranian sites, officials and documents for its investigation.
Iranian Ambassador Reza Najafi, leading the Islamic Republic's negotiating team for the first time since his appointment last month, said he hoped for an agreement soon.
"We, indeed, should continue these constructive discussions and we hope that we could reach an agreement as soon as possible," he told reporters, standing next to Nackaerts.
For several years, the IAEA has been investigating suspicions that Iran may have coordinated efforts to process uranium, test explosives and revamp a ballistic missile cone in a way suitable for a nuclear warhead.
Iran says the allegations are baseless, but has pledged, since Rouhani took office in early August, to expand cooperation with the U.N. agency. Western diplomats have accused Iran of obstructing the IAEA investigation in the past.
Israel, believed to be the only nuclear-armed state in the Middle East, has said Iran's new, conciliatory approach is merely an attempt to "buy time" to push ahead with its nuclear work without fear of military action.
U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif, who met privately in New York on Thursday as well as in talks with other major powers about the nuclear dispute, both expressed cautious optimism.
Iran says its programme is a peaceful, but its refusal to curb sensitive nuclear work and lack of full openness with IAEA inspectors have drawn tough Western sanctions, hurting its lifeline oil exports.
Rouhani said this week that Iran would never develop nuclear weapons and called for a nuclear deal in three to six months.