* Nuclear dispute with Iran has persisted for years
* New Iranian president seen as "opportunity"
* Has long history of involvement in the issue
Six world powers see a chance that Iran's relatively moderate new president, beset by sanctions and worried about unrest in the region trickling home, may be more amenable to compromise in a long-standing nuclear dispute, a senior Western diplomat said.
The powers - Russia, China, France, Britain, Germany and the United States - are hoping years of economic pressure will finally force Iran to scale back its nuclear work, which they suspect aims at allowing Tehran to make bombs.
President-elect Hassan Rouhani won the June 14 election largely on a promise of repairing the economy, and will be under pressure in the coming months to deliver, the diplomat said.
He would be mindful that without progress and at least some relief from sanctions, which would require the scaling back of Iran's nuclear work, his public support could quickly wane.
"Rouhani's election could provide an opportunity and we are expecting to see a change in tone," the diplomat said, who spoke on condition of anonymity but has close knowledge of the issue.
The comments follow a meeting of senior officials from the six nations in Brussels this week, convened to devise a strategy for the future of nuclear diplomacy after the Iranian election.
"Iran is also looking carefully at what is happening in its neighbourhood, especially in Egypt and in Syria," the diplomat said.
Adding some scepticism voiced by negotiators from the six powers, the diplomat said: "We will have to judge the new Iranian government by its actions."
The six powers said this week they hope to resume talks "as soon as possible", and several diplomats said a new round of talks could take place in September, possibly before the annual meeting of the United Nations General Assembly in New York.
The last round, held in April, produced no breakthrough, after Iran rejected an offer of some sanctions relief.
The powers had asked, in return, that Tehran abandon enriching uranium to 20 percent fissile purity - a process that closes a major technological gap on the way to making weapons-grade material - as well as ship out an existing stockpile and close a facility where such work is done.
Tehran denies its work has any military aims and wants major economic sanctions to be lifted before it curbs the programme.
The diplomat said Rouhani's past as an Iranian nuclear negotiator meant he would likely be more involved in nuclear diplomacy than his predecessor, Mahmoud Ahmadinejad.
Rouhani presided over talks with Britain, France and Germany that saw Tehran agree in 2003 to suspend uranium enrichment-related activities pending further negotiations on trade and diplomatic concessions to Iran, ultimately undone by mutual mistrust.
He resigned after Ahmadinejad took office in August 2005; enrichment activity later resumed and expanded.
"He has a particular interest in the nuclear file and knows it extremely well because it is central to his career," the diplomat said.
Another Western diplomat said Rouhani's election was seen among the six nations as "the best possible outcome". But he voiced concerns that Rouhani's potentially more conciliatory tone could disguise inaction.
"It is easier to create positive impressions without practical results," he told Reuters.
Without progress in the nuclear dispute, Israel has threatened to bomb the Iranian nuclear installations, and international negotiators are keen avoid any military action that could inflame the region.
Israel is widely believed to be the only nuclear power in the Middle East and sees Iran as an existential threat.
Western concerns have been reinforced recently by Tehran's plans to launch a new heavy water reactor in 2014.
Western diplomats and experts say the Arak reactor could yield plutonium for bombs if its spent fuel were reprocessed, something which Iran says it has no intention of doing.
"We are concerned about continuing expansion of Iranian nuclear activities, including the plutonium track. The construction of the Arak heavy-water reactor is accelerating," the senior diplomat said.