TEHRAN, Iran -- Iranian nuclear technicians set dozens of centrifuges spinning Tuesday to begin enriching uranium stocks to a significantly higher level, prompting President Barack Obama to warn of a "significant regime of sanctions."
Iran's acceleration in its enrichment program was a defiant step that puts weapons-grade uranium in closer reach, should Tehran choose to go after the bomb. It was also another in a series of mixed messages that appeared calculated to boost Iran's leverage in negotiations with world powers on limiting its nuclear program.
Only days ago, President Mahmoud Ahmadinejad said Iran was ready to send its stockpiled uranium -- currently enriched to safer, low levels -- to Russia and France for processing into fuel rods to power a research reactor. That was proposed last year in a U.N.-drafted plan aimed at eliminating Iran's ability to enrich those stocks to higher levels needed for making warheads.
Obama, speaking at a surprise appearance in the White house briefing room, said the sanctions process is moving along quickly, but he gave no specific timeline.
In his most extensive remarks on Iran in some time, Obama said Iran appeared to have spurned his offer of engagement.
But even announcing its latest step, Iran was careful to leave the door open to a negotiated solution, saying it would stop the work if the West found a way to provide it with fuel for the research reactor, which makes radio isotopes for use in cancer treatment.
"Whenever they provide the fuel, we will halt production of 20 percent," Ali Akbar Salehi, the head of Iran's nuclear program, told state TV late Monday.
He was referring to the level of enrichment necessary for the Tehran reactor. Iran's current stockpile has been spun to a level of 3.5 percent, suitable for use in fueling power plants, which is Iran's primary stated aim for its enrichment program.
The United States and its allies in Europe suspect Iran is using such civilian work to mask an effort to develop a weapons capability. Central to their concern is uranium enrichment, which at levels of 90 percent provide a possible pathway to nuclear arms production.
Iran denies it wants to produce weapons but has defied U.N. resolutions demanding it halt enrichment as a way of easing the concerns.
Even before the announcement, U.S. Defense Secretary Robert Gates said he believed the U.N. should slap new sanctions on Iran in "weeks, not months," according to his spokesman.
France also said Iran's action left no choice but to push harder for a fourth set of U.N. Security Council sanctions to punish Iran's nuclear defiance.
Even Russia, which has friendly ties with Iran and has opposed new sanctions, appeared to edge closer to other Security Council members supporting harder penalties, saying the enrichment work has raised new suspicions.
"Iran says it doesn't want to have nuclear weapons. But its actions, including its decision to enrich uranium to 20 percent, have raised doubts among other nations, and these doubts are quite well-founded," said Nikolai Patrushev, the chief of Russia's Security Council.
Senior diplomats from the six major powers -- the U.S., Russia, China, Britain, France and Germany -- held a conference call last Friday to discuss a U.S.-produced list of possible new sanctions. U.N. diplomats briefed on the talks, speaking on condition of anonymity because they were private, said no decisions were made.
Diplomats say China holds the key to new sanctions and Tuesday's statement from its Foreign Ministry calling for more talks indicates that Beijing is not yet ready to impose tough new measures.
Iranian state television said the process began Tuesday in the presence of inspectors from the U.N.'s International Atomic Energy Agency, which has investigated Iran's nuclear program since 2003.
In Vienna, where the agency is headquartered, a senior U.S. envoy questioned the rationale for Tehran's move and offered Washington's help in getting the medical isotopes needed to treat cancer patients in Iran.
Iran says more than 850,000 people need the isotopes and radiography materials produced by the Tehran reactor for their illnesses.
"To address the humanitarian needs of Iran's people, we are prepared to facilitate Iran's procurement of medical isotopes from third-country sources," said Glyn Davies, the chief U.S. delegate to the IAEA.
Davies told The Associated Press that the U.S. proposal represented "a faster, cheaper and more responsible alternative" than enriching to 20 percent -- a move he described as "a provocation ... in defiance of U.N. Security Council resolutions."
Iran does not have the technology to turn the higher-grade uranium into the metal fuel rods needed for the Tehran reactor, and Davies said its actions "call into question" its true motivation.
Salehi said, however, that time will show Iran can make the fuel rods at a facility in the city of Isfahan. "Wait and see," he said.
Iran has been trying to buy the more highly enriched fuel for its research reactor for the past several months, Salehi said, but the West made providing the fuel conditional on Iran's acceptance of the U.N.-drafted agreement to ship its uranium stockpile abroad first.
Tuesday's work began with Iranian scientists injecting 55 pounds (25 kilograms) of 3.5 percent enriched uranium hexafluoride gas into a cascade of 164 centrifuge machines at a laboratory in the central town of Natanz, state TV reported.
The machines are expected to produce about 5.5 pounds (2.5 kilograms) of 20 percent enriched uranium out of 55 pounds of gas every month, according to the report.
U.N. nuclear inspectors already overseeing enrichment to low levels were allowed to stay on site to monitor the process, state TV said.
Gill Tudor, a spokeswoman for the U.N. agency, said only that it had inspectors in the country already conducting "normal safeguard operations."
Although material for the fissile core of a nuclear warhead must be enriched to a level of 90 percent or more, just getting its stockpile to the 20 percent mark would be a major step for Iran. While enriching to 20 percent would take about one year and require up to 2,000 centrifuges, any next step -- moving from 20 to 90 percent -- would take only six months and between 500-1,000 centrifuges.
Reflecting international concern, 44 Nobel Prize winners, including Elie Wiesel, published a letter in the International Herald Tribune urging Obama and other world leaders to implement harsher sanctions and take steps to stop Iran's nuclear work as well as its crackdown on pro-reform protesters.
Iran's Foreign Ministry spokesman Ramin Mehmanparast said any plan by the West to impose new Security Council resolutions would be a "mistake."
"They are completely wrong if they think our people will back down even a single step," he said.