LAUSANNE, Switzerland – French diplomats are questioning optimism expressed by Iranian officials in recent days that negotiations over its nuclear program have neared a critical breakthrough.Iranian Atomic Energy Organization chief Ali Akbar Salehi’s assertion that 90 percent of technical issues in the talks have been agreed upon is “not correct,” one European official told journalists here on Thursday.French Foreign Minister Laurent Fabius has been the most vocally critical of the state of talks in recent days.“Contrary to what many think, that we are only discussing one issue, it is not correct,” the diplomat told reporters. “We are discussing many issues and nothing is agreed until everything is agreed.”He said research and development on centrifuges is one of many issues still log-jamming the talks.“Research and development currently has become the most critical and difficult issue, and there won’t be an accord if the Iranians don’t back down,” he said, adding that political issues, not just technical, are slowing progress.US Secretary of State John Kerry was more measured and conciliatory in his remarks.“We are pushing some tough issues, but we made progress,” Kerry told traveling press here. He declined to comment on Israel’s elections.Technical experts from the US, UK, France, Russia, China and Germany continued intensive discussions with Iran throughout the day, and Kerry continued his direct talks with Iranian Foreign Minister Mohammad Javad Zarif.US officials swiftly denied reports suggesting a deal had been cut on the number of uranium-enriching centrifuges Iran will be allowed to retain.“The fundamental framework issues are still under comprehensive discussion,” one senior State Department official said. “There is no draft document being circulated.”Media organizations in Lausanne grew in number on Thursday as talks intensified at the Beau Rivage Palace, overlooking Lake Geneva on the Swiss Riviera. But talks are expected to extend beyond the weekend, when the Persian new year begins, until a March 31 deadline for a political framework agreement.A senior US Treasury official said Thursday the US will remove sanctions on Iran only in stages as part of a nuclear deal, tied to Tehran’s “verifiable” steps in curbing its nuclear activities.If the powers reach a deal, Washington still plans to keep any sanctions tied to Iran’s support for militant groups, human rights abuses and other “destabilizing” activities in the Middle East, said Adam Szubin, the acting head of Treasury’s Office of Terrorism and Financial Intelligence.Reuters contributed to this report.