Mastering enrichment will move Tehran a big step closer to being able to build an atomic bomb. Iran's progress already has intensified its confrontation with the United States and other countries that fear it is trying to develop nuclear weapons.
Despite persistent suspicions, however, a report due next month by the U.N. nuclear watchdog agency is not expected to provide proof that Tehran has a weapons program, diplomats said.
Nearly two years of inspections have uncovered a pattern of concealment and deception by Iran over two decades. But when it comes to whether Iran is secretly pursuing an atomic bomb, the case remains circumstantial.
Iran insists that its goal is to generate electricity. Its leaders have so far rejected demands by the U.N.'s International Atomic Energy Agency, the U.S. and European countries that they freeze enrichment activities.
A showdown appears to be approaching. The U.S. and its allies, arguing that the threat is imminent, want the U.N. Security Council to impose sanctions on Iran for violating the Nuclear Nonproliferation Treaty, which limits the spread of nuclear technology to peaceful purposes.
But since the United States failed to prove its claims about the existence of weapons of mass destruction in Iraq, other countries want more time for a fuller evaluation of Iran's enrichment capabilities and intentions.
At the center of the dispute is the enrichment process itself, which converts uranium ore into fissionable material, the most elusive component of a nuclear weapon. The same basic process produces low-level enriched uranium for civilian reactors or, with technical adjustments, highly enriched uranium for bombs.
This month, Iran said it was gearing up to produce large amounts of gaseous uranium, which is used in enrichment. The gas, known as uranium hexafluoride, can be fed into slender centrifuges, which spin at high speed to transform the gas into enriched uranium.
Iran has moved much faster than expected in manufacturing and assembling these centrifuges, diplomats said. The rapid progress means a pilot centrifuge plant near Natanz, in central Iran, could soon be equipped with enough machines to begin large-scale enrichment.
Two senior European diplomats said the pilot plant could be expanded from the existing 164 centrifuges to 1,000 within weeks and produce enough material in less than a year to fashion a crude nuclear device.
"They need to install more centrifuges and do preparatory work, and they could be in production in shorter than a year," said one diplomat, who, like most of the people interviewed for this article, spoke on the condition that his name and position be withheld.
For now, the International Atomic Energy Agency is monitoring the gas-production plant at Esfahan, also in central Iran, and preparations at the pilot plant. The pilot operation is part of a complex where an underground enrichment facility for as many as 50,000 centrifuges is under construction.
Western intelligence officials said the big fear is that once the two plants are operating, Iran will shift enrichment operations to hidden installations or follow North Korea's example and kick out the IAEA, allowing Tehran to begin enriching uranium to weapons grade at Natanz.
Uranium enrichment is relatively portable. Experts say 1,000 centrifuges could operate in a small building with little chance of detection by even the most sophisticated sensors or satellites.
There is no evidence that a hidden plant exists, and only hints about weapons research. But even officials who give Iran the benefit of the doubt say Tehran has been caught in so many lies that verifying the absence of a weapons program would take months, if not years, and might be impossible.
"When people have looked you literally in the eye across the table and told you this is black and it turns out to be white, your confidence in them is damaged," said a third senior European diplomat.
Mohamed ElBaradei, the IAEA director-general, has praised Iran's cooperation often enough to evoke U.S. anger, but he also has acknowledged that Iran's actions have created a "deficit of confidence." As before the Iraq war, ElBaradei wants more time to complete inspections before sending the matter to the Security Council.
But the U.S. and allies such as Canada and Australia say time has run out. They argue that the threshold for action is not the discovery of a secret plant or a weapons design. Instead, they say, Iran must be stopped before it begins to enrich uranium.