(Adds U.S. reaction)
By Louis Charbonneau
UNITED NATIONS, Jan 8 (Reuters) - Negotiations between Iran and six world powers on implementing a landmark November deal to freeze parts of Tehran's nuclear program in exchange for easing some sanctions have run into problems over advanced centrifuge research, diplomats said.
The dispute over centrifuges highlighted the huge challenges facing Iran and the six powers in negotiating the precise terms of the Nov. 24 interim agreement. If they succeed, they plan to start talks on a long-term deal to resolve a more than decade-long dispute over Tehran's nuclear ambitions.
Among the issues to be resolved in political discussions due to begin in Geneva later this week is that of research and development of a new model of advanced nuclear centrifuge that Iran says it has installed, diplomats said on condition of anonymity.
Centrifuges are machines that purify uranium for use as fuel in atomic power plants or, if purified to a high level, weapons.
"This issue (centrifuges) was among the main factors in stopping the previous technical discussions on Dec. 19-21," a Western diplomat told Reuters on condition of anonymity.
Other Western diplomats confirmed that centrifuges remained a "sticking point" in the talks with Iran but noted that last month's discussions were understandably adjourned ahead of the December holidays - not because of the centrifuge issue.
"As part of the (Nov. 24) agreement, Iran is permitted to engage in R&D (research and development), but that is tempered by the fact that it is prohibited to install new centrifuges, except as required by wear and tear," the first diplomat said.
State Department spokeswoman Jen Psaki said the United States was keen to see the interim deal implemented, though she declined to predict the outcome of the latest talks.
She said U.S. Undersecretary of State for Political Affairs Wendy Sherman will be in Geneva on Thursday to discuss the issue with her European Union counterpart, Helga Schmid, and Iran's negotiator, Deputy Foreign Minister Abbas Araqchi.
In December, Al-Monitor, a news website focusing on the Middle East, cited a former U.S. official as saying Iran had notified the six powers it wanted to install additional "IR-2m" centrifuges, modified versions of second-generation machines. The website also said the former U.S. official suggested this may have played a role in the dispute.
But diplomats now say Iran has told the six countries it wants to press ahead with the development of even more advanced centrifuges than the IR-2m.
Iran is already testing several different new, more efficient centrifuge models at its Natanz research facility, according to the U.N. nuclear watchdog. Iran's statements last month that it was testing a new advanced centrifuge have not made clear whether it is an entirely new model or a modified version of an installed one.
'SERIOUS NUCLEAR NEGOTIATIONS'
Western diplomats said they were uncomfortable with the idea of Iran pressing ahead with the development of more advanced centrifuges. But Iran says centrifuge research is crucial.
"We have to make sure our right to research and development is respected," a senior Iranian government official said on condition of anonymity.
The research and development would be aimed at improving Iran's existing centrifuge technology so it can enrich better and faster, a prospect Western governments find worrisome.
Iran and the so-called P5+1 - the United States, France, Britain, Germany, Russia and China - reached a deal on Nov. 24 in Geneva aimed at curbing the Islamic Republic's most sensitive nuclear work, including medium-level 20 percent uranium enrichment, in return for easing some economic sanctions.
Iran is under U.N., U.S. and European Union sanctions for refusing to heed U.N. Security Council demands that it halt all enrichment- and plutonium-related work at its nuclear sites. Tehran rejects Western allegations that it is seeking the capability to produce atomic weapons, saying its nuclear ambitions are limited to the peaceful generation of electricity.
Nuclear experts from Iran and the six powers have held several rounds of talks since Nov. 24 to resolve various technical issues before the interim deal can be put into place.
The experts have to work out when the accord will be implemented. Western diplomats and Iranian officials say the six powers and Iran want to start implementing the deal on Jan. 20.
A senior Western diplomat, however, said that despite the disagreements, the latest rounds of talks between Iran and the six powers "actually made pretty good progress."
"There are still, however, some outstanding issues. But we are still aiming to get the interim agreement started on Jan. 20," he said "We haven't given up hope of that."
Iran's Foreign Minister Mohammad Javad Zarif said on his Facebook page that Tehran was "very serious" about the Geneva deal. "Serious nuclear negotiations are under way and with strong political will," he added.
Diplomats said the push by Iran to continue advanced centrifuge research and the resistance by Western powers to the idea is not surprising given what it is at stake for all sides.
"The gaps which have been making it difficult to reach an agreement clearly reflect the attempts of both sides to improve their status at the last stage before signing the agreement," the first diplomat said.
"Iran seeks maximum maneuvering room in interpreting the agreement, while the U.S. seeks to ensure that this interpretation does not go beyond its understanding of the agreement," he said. "Either way, the two sides are interested in reaching an agreement as soon as possible."
Israel, which has been highly critical of the six powers' deal with Iran, was not surprised by Iran's attempts to ensure that it could continue with advanced centrifuge research.
"It was clear from the outset that the Iranians would play games," an Israeli official said on condition of anonymity. "They did it in the past, and now they're up to their old tricks again."
Iran's negotiations with Britain, France and Germany in 2003-2005 collapsed after Tehran and the European trio failed to agree on what enrichment-related activities would be permitted under a voluntary suspension of the Iranian enrichment program.
The Europeans accused Tehran at the time of violating the terms of the suspension while Iran said London, Paris and Berlin failed to deliver promised economic incentives and kept trying to expand the scope of the agreed freeze. (Additional reporting by Fredrik Dahl in Vienna, Justyna Pawlak in Brussels, Parisa Hafezi in Ankara, Dan Williams in Jerusalem, David Brunnstrom in Washington; Editing by Cynthia Osterman and Jonathan Oatis)