VIENNA (AP) -- Nearly seven weeks after signing a landmark nuclear deal, Iran and six world powers hope to reach an agreement this week on its implementation. But differences over Tehran's push to improve its uranium enrichment abilities could delay its enactment and strengthen critics of the accord in Washington and Tehran.
Both sides say agreement is possible at a two-day meeting beginning Thursday afternoon in Geneva -- with caveats.
European Union spokeswoman Maja Kocijanic said "some issues remain to be resolved" during the talks, a statement echoed by Iranian Foreign Ministry spokeswoman Marzieh Afkham.
The meeting is formally being held by Iranian Deputy Foreign Minister Abbas Araghchi and EU senior negotiator Helga Schmid on behalf of the six world powers.
But the U.S. State Department has announced that senior U.S. negotiator Wendy Sherman would also attend -- an indication that Washington wants to be in place if the two sides overcome their differences and announce a deal by Friday.
Asked if the United States anticipated agreement, State Department spokeswoman Jen Psaki said Wednesday that "there are just a few remaining issues, so we're working toward that."
Psaki did not go into specifics on the nature of the disagreements in keeping with the confidential nature of the negotiations.
But two officials told The Associated Press that Iran is coming to the table with demands to exempt a facility used for research and the development of uranium enrichment from the overall curbs on its enrichment. That is something opposed by the six powers which sealed the Nov. 24 deal with Tehran.
Depending on its grade, enriched uranium can be used either for reactor fuel or -- at levels above 90 percent -- for the fissile core of a nuclear warhead. Iran insists it has no interest in nuclear weapons only nuclear power but the United States and its allies are skeptical.
Limiting uranium enrichment is one of the core aims of the six-month interim deal meant to prepare ground for a permanent accord on Iran's nuclear program.
Under the November agreement, Iran agreed to limit its uranium enrichment to 5 percent -- the grade commonly used to power reactors. The deal also commits Iran to stop producing 20 percent enriched uranium -- which is only a technical step away from weapons-grade material -- and to neutralize its 20 percent stockpile.
At the same time, the agreement allows Tehran to continue enrichment research and development -- a loophole the two officials say Iran interprets as allowing it to continue producing 20 percent uranium at its research and development site at Natanz, south of Tehran.
Iranian negotiators say no additional 20 percent material will be accumulated, because any made at the site will be immediately neutralized, said the officials, who represent countries that are members of the Vienna-based U.N. nuclear agency monitoring Tehran's atomic activities. They spoke on condition of anonymity because they are not authorized to discuss what is said at the closed meetings.
But representatives of the six powers -- the United States, Russia, China, Britain, France and Germany -- argue that the preliminary Geneva deal prohibits all enrichment above 5 percent, even for research and development purposes.
The two sides also are coming to the table Thursday with an additional dispute about what can be done at the Natanz site. As reported by the AP last month, Iran told representatives of the six powers that it had installed some advanced centrifuges at the facility after signing the Nov. 24 deal, asserting that it had a right to do so under the research and development provisions of the accord.
That is being opposed by the United States and its allies. They argue that installing any centrifuge that increases overall numbers, particularly a new model, violates Tehran's commitment to freeze the amount and type of enriching machines it has at Nov. 24 levels.
When the AP sought comment on the topic, the office of Reza Najafi, Iran's chief IAEA delegate, said he was not available to speak to non-Iranian media. Calls to Iranian officials in Tehran were not immediately returned.
Also Thursday, the Kremlin announced that Russian President Vladimir Putin had spoken by telephone with Hassan Rouhani, his Iranian counterpart, about the issues surrounding implementation of the Geneva agreement.
The disagreements reflect the difficulties in implementing the Nov. 24 deal, which outlines both Iran's obligations and moves by the international community to ease economic sanctions in return for Tehran's nuclear concessions.
Both sides have said they hope the interim agreement can come into force by late January.