Iran four months ago discreetly contacted the United Nations-affiliated agency for nuclear energy to outline a worrisome situation: A research reactor in Tehran that produces medical isotopes that detect and treat the diseases of about 10,000 patients a week will run out of fuel by the end of 2010. Iran also had a request: Can you help us find a country that will sell us new fuel?…

Now the Iranian request is at the center of an unusual deal, brokered largely by the United States, that aims to buy time for a diplomatic solution to the impasse over Iran’s nuclear ambitions. If it works, Iran will end up with fuel necessary to treat desperately ill patients — and greatly reduce its stock of low-enriched uranium. But critics question why the United States would be assisting a nuclear pariah — and giving it fuel that is even more enriched than its current holdings — without even an agreement that Iran stop operating a uranium enrichment facility in violation of numerous U.N. Security Council resolutions.

“You aren’t buying much more time,” said Henry Sokolski, executive director of the Nonproliferation Policy Education Center in Washington. “You are bailing water out of a sinking ship rather than plugging a hole.”…

“This is a real confidence-building measure,” said a senior administration official, speaking on the condition of anonymity because of diplomatic sensitivities. “If they say they need it for medical purposes, we are offering it to them. If they accept it, it is LEU [low-enriched uranium] coming out. If they reject it, it is another data point that says, ‘Look, these guys are not serious.’ “