Tehran could soon have humankind’s most frightening weapon if substantial diplomatic progress is not made in the coming days.

For years, Iran spurned diplomatic overtures to address the threat posed by its nuclear program. Meanwhile, its efforts have progressed to the point that it already has enough low-enriched uranium to produce fuel for at least one nuclear bomb.

Tehran’s admission this week that it has secretly constructed a second enrichment plant suggests that its program may be further along than we had imagined. We do not have much time to wait.

I support President Obama’s efforts to engage Iran. Thanks to these efforts, no one will be able to say that we failed to do everything possible to give Iran a diplomatic way out. But there is more than ample reason to be skeptical that the regime in Tehran intends to come clean about its nuclear program.

It is critical that we set clear timelines and benchmarks by which to judge Iranian intentions as well as unambiguous consequences if Iran fails to meet the criteria. The window for Iran to demonstrate seriousness of purpose should start with the Oct. 1 meeting and, as Obama has indicated, should close by the end of the year.

If Tehran is serious about engagement, it should agree early on to meaningful steps, such as a “freeze for freeze” in which Iran does not add to its enrichment capabilities — including halting construction on the second enrichment facility, as verified by the International Atomic Energy Agency — in exchange for an agreement that no additional international sanctions would be imposed during this period. Iran must also agree to verifiably suspend nuclear enrichment by year’s end. Were that to happen, the international community could enter into detailed negotiations with Iran about all issues of concern and the incentives that could be offered in exchange for a satisfactory understanding of Iran’s nuclear intentions and assurance that Iran would not be able to acquire a nuclear weapons capability.

But if, as I expect, that scenario does not come to pass, we should be ready immediately to impose what Secretary of State Hillary Rodham Clinton has called “crippling sanctions.” Iran’s economy is in terrible shape, and the regime no longer can take for granted the support of its citizens. The best conduit for such sanctions would be a mandatory U.N. Security Council resolution.

To have a sanction bill ready for the president’s signature by early next year, we must start the process for passing it now. Should negotiations with Iran not succeed and should multilateral sanctions not get off the ground, we must be prepared to do what we can on our own.

Howard Berman is a Democratic representative from California and the chairman of the House Foreign Affairs Committee. A version of this commentary was originally published in the Washington Post.