* Iran-IAEA pact struck to help allay nuclear concerns
* Complements Iran's deal with powers to curb atomic work
* IAEA has lacked access to heavy-water plant since 2011
* UN inspectors to expand monitoring of Iran nuclear sites
The U.N. atomic watchdog will probably need more money to verify the implementation of a landmark nuclear deal between Iran and six world powers, its chief said on Thursday, and it would take some time to prepare for the task.
Yukiya Amano also said Iran has invited the agency to visit the Arak heavy-water production plant on Dec. 8, the first concrete step under a new cooperation pact aimed at clarifying concerns about the Islamic Republic's nuclear programme.
Both agreements indicate how Iran is acting quickly to address fears about its nuclear programme after the election in June of a relative moderate, Hassan Rouhani, as new president on a platform to smooth its troubled relations with the world.
The International Atomic Energy Agency can mobilise expertise and staff from within the organisation for an increased workload in checking whether Iran is complying with the interim accord with the major powers to curb its nuclear programme, IAEA Director General Amano told a news conference.
But its budget is very tight, he added: "Naturally this requires a significant amount of money and manpower ... I don't think we can cover everything by our own budget."
The Arak facility produces heavy water intended for use in a nearby research reactor that is under construction. The West is concerned that the reactor, which Iran has said could start up next year, could yield plutonium as fuel for atomic bombs once operational. Iran says it will make medical isotopes only.
As part of its agreement with the powers, Iran is to halt installation work at the reactor and stop making fuel for it.
The IAEA will need to expand monitoring of Iran's uranium enrichment plants and other sites under the Nov. 24 breakthrough deal reached after marathon talks between Iran and the United States, Russia, China, France, Germany and Britain.
The IAEA was studying how to put into practice the agreement with respect to its inspectors' role in checking compliance and this would take "some time," Amano said, adding it was a complicated task that needed proper preparations.
"This (analysis) will include the implications for funding and staffing," he separately told the IAEA's 35-nation board.
About 10 percent of its annual 121-million-euro ($164 million) budget for inspections is already devoted to Iran. The agency has two to four staff in Iran virtually every day of the year, with some 20 dedicated to inspector activity there.
Under the Geneva interim accord, there will be "significant extra work and they will require extra resources to do it," a Western envoy said, with "the extremely complex and difficult implementation" expected to start in January.
The agreement between Iran and the powers is designed to halt any further advances in Iran's nuclear campaign and buy time for talks on a final settlement of the decade-old dispute.
After years of confrontation, it underlined a thaw in relations between Iran and the West after the election of Rouhani on a pledge to end Tehran's isolation and win relief from sanctions that have battered the oil producer's economy.
"DEVIL IN THE DETAIL"
But Western officials and experts caution that finding a permanent solution to the Iranian nuclear issue will probably be an uphill struggle, with the two sides still far apart on the final scope and capacity of the Iranian nuclear programme.
The Islamic Republic says it is a peaceful energy project but the United States and its allies suspect it has been aimed at developing the capability to produce nuclear weapons.
Iran agreed on Sunday to stop its must sensitive nuclear work - uranium enrichment to a higher fissile concentration of 20 percent - and cap other parts of its activities in exchange for limited sanctions relief.
Refined uranium can fuel nuclear power plants but also the fissile core of a bomb if processed to a high degree.
"The IAEA inspectors are able to give an early warning if Iran does not comply at these locations with its undertakings," former IAEA chief inspector Olli Heinonen said. "In verification work, the devil is in the detail."
The IAEA's visit in 10 days' time to the heavy water production plant near the town of Arak is part of a separate agreement signed this month between the U.N. agency and Iran.
Inspectors have not been there since August 2011, despite repeated requests. But Iran agreed on Nov. 11 to grant access to this site and to a uranium mine within three months.