Secretary of State Hillary Rodham Clinton dismissed Iran's proposal, a hastily negotiated deal with Turkey and Brazil, as a "transparent ploy" to head off the U.N. sanctions she has been painstakingly pursuing for months.

And she was right. But that doesn't mean the United States and its allies should say no to the deal. Instead, they should figure out a way to pocket Iran's offer and challenge aspiring world powers Turkey and Brazil to make it better.

At the moment, the administration is focused on getting its sanctions resolution through the U.N. Security Council, and then, equally important, persuading as many countries as possible to impose additional sanctions that go beyond the relatively mild language of the resolution.

But as one official involved in the policy reminded me this week, sanctions aren't the aim of U.S. policy on Iran; they're only a means to an end. The real goal is inducing Iran to stop moving in the direction of nuclear weapons, which means halting its enrichment of uranium to the level needed to make a bomb.

That's why, in October, the United States and its allies offered Iran a deal: If the Tehran regime would swap most of its low-enriched uranium for highly enriched (but safeguarded) uranium for medical use, the push for more sanctions would be called off. The idea was to give the Iranians a face-saving way of slowing their progress toward nuclear weapons capability, without forcing them to say so explicitly.

Initially, Ahmedinejad said yes, but almost immediately his experiment in compromise ran afoul of Iranian domestic politics. The president's rivals, including some leaders of the purportedly moderate opposition, quickly attacked him for going soft on the Great Satan. Ahmedinejad briefly defended the deal, then accused the United States of poisoning the well by portraying it as a defeat for Iran — and backed down.

The lesson many Western officials took from the negotiations was that the old stereotype must be true: The Iranians are unreliable negotiating partners.

Since then, the picture has only gotten worse. Iran has continued enriching uranium, and it has defiantly declared that it plans to keep doing so even as it denies (without much credibility) that it intends to make nuclear weapons.

That's where things stood when Turkey and Brazil entered the picture. The deal they negotiated with Iran, on its face, looked similar to what the United States offered last fall: Ahmedinejad agreed to ship 2,600 pounds of nuclear fuel to Turkey in a swap for the medical uranium he says he needs.

But Clinton and other U.S. officials pointed out, correctly, that shipping out 2,600 pounds this year wouldn't slow Iran's program as much as it would have last year, because the Iranians now have larger stores of enriched uranium.

"The whole idea was to put more time on the clock, and this deal doesn't get you as much clock," a U.S. official said. And the Iranians paired their offer with a defiant declaration that they still intend to continue enriching other fuel.

Nevertheless, some officials in the Obama administration say that the Turkish-Brazilian deal might be the start of something significant.

"We're looking at it," one official said. "In its current form, it's only a half measure.... But if we can get assurance that the sanctions resolution is going to pass, and if the Iranians are able to sit down and go further" — meaning increase the amount of uranium they are willing to give up — "then it becomes interesting."

So the administration's first move has been to make sure its hard-built coalition in favor of stepped-up sanctions, including Russia and China, would hold together. (That was a major item on Clinton's agenda during her visit to Beijing this week, and one reason her tone there was so tough.)

But its second move, now under discussion, may be to endorse Iran's new offer to ship much of its uranium to Turkey — if the plan can be strengthened to slow Iran's uranium enrichment program in a verifiable way.

"The temptation is to do what we've been doing for 30 years, which is to dismiss everything the Iranians put forward. You have to dismiss that temptation," one official said.

A deal the Iranians rejected when it came from the United States might be more palatable when it's sponsored by Turkey and Brazil, or so the diplomats' logic goes.

It's still a long shot. But it's a low-risk venture, well worth trying. The United States spent decades striking nuclear agreements with a repressive Soviet Union even as it supported efforts to democratize the Soviet bloc. We need to revive that uncomfortable combination of approaches as we wrestle with Iran.