Dennis Ross, a counselor at the Washington Institute for Near East Policy, was a senior Middle East adviser to President Obama from 2009 to 2011. David Makovsky is a senior fellow at the Washington Institute.

As the conflict in Syria rivets international attention, Iran’s nuclear program continues apace. Unfortunately, while the Iranians install the next generation of centrifuges — machines that can produce enriched uranium three to four times faster than before — the “P5+1” negotiations on Iran’s nuclear program have ground once again to a halt.

While economic pressures impose a cost on Iran, so far they have failed to alter its nuclear program. Ayatollah Ali Khamenei may acknowledge that sanctions are “brutal,” but he also seems to feel that Iran has endured worse. In light of President Obama’s objective of preventing the Iranians from acquiring nuclear weapons, something has to give. At a minimum, the supreme leader must be made to feel that when the United States says the time for diplomacy is running out, we mean it — and that the consequence is likely to be the use of force.

Perhaps because of U.S. hesitancy on Syria, or our withdrawal from Iraq, or our transition out of Afghanistan, or talk of the U.S. “pivot” to Asia, Iranian leaders seem not to believe that we will use force if diplomatic efforts fail. Obama insists that he means what he says on preventing Iran from having nuclear weapons and that he will do whatever is necessary. The Iranian misreading of this determination could put us on a fast track to conflict.

If diplomacy is to be given a final chance, the United States needs to shift its negotiating strategy away from the confidence-building “step-by-step” approach — which only deepens Iranian perceptions that they can string us along until we acquiesce. Instead, the United States needs to establish greater clarity about what we can and cannot live with regarding Iran’s nuclear program and give further credence to the administration’s statements that the time for diplomacy is running out.

The confidence-building approach, which seeks to reach a limited agreement in a bid to buy time for a wider deal in the future, simply cannot do that. Even if it were possible now, it is not clear that such a tactic would be in U.S. interests. A limited deal is based on the notion that capping Iran’s “medium-enriched” uranium at 20 percent enrichment will guard against it being able to upgrade its fuel to weapons-grade enrichment. Yet if Iran has a bomb’s worth of uranium enriched to 20 percent, it would take only 30 to 40 days for it to produce weapons-grade fuel.

With Iran expanding its number of first- and second-generation centrifuges, even if its medium-enriched uranium were capped or shipped out of the country as part of some international agreement, the Iranians could surge to weapons-grade almost as fast with their four to five bombs’ worth of low-enriched uranium.

Iran continues to stall negotiations under the cover of not ostensibly crossing a “red line.” The United States and its allies must change gears. It may be best to do so before Iran’s June 14 elections — not because a deal is likely to be reached before the vote but because the Iranians will need time to contemplate the meaning of an approach geared more toward a nuclear endgame.

This new approach would involve defining an acceptable civil nuclear capability for Iran — something that the confidence-building approach has largely avoided. It could mean accepting limited enrichment but with strict and verifiable restrictions. This would prevent Iran from being able to break out and present the world with a nuclear weapons fait accompli.

Practically, there would need to be limits on the number and type of centrifuges, maximum level of enrichment and amount of enriched uranium that could remain in Iran. Each of these amounts would have to be small. Clearly, if Iran is prepared to alter its nuclear program in this fashion, we should be prepared to lift the harsh economic sanctions. But the Iranians cannot get the latter unless they do the former.

Apart from taking away Iranian excuses, an endgame approach to the nuclear issue also has the benefit of creating far greater clarity in Iranian minds. It would signal that we mean what we say — that time is indeed running out. By offering Iran what its leaders have claimed to want, civil nuclear power, the United States could expose Iran’s true intentions to the world, including its own people. Were Iranian leaders to turn down the opportunity to have civil nuclear capability, their real aims of acquiring nuclear weapons would be revealed. In such circumstances, the United States would be far better positioned to make the case to the international community that military action is warranted.

Coercive diplomacy succeeds when threats are believed and the game-playing and manipulation stop. Offering a credible endgame proposal could convince the Iranians that time is truly running out — and that we are setting the stage for the use of force if diplomacy fails. We should give Iran a clear diplomatic way out — and Iranians should understand the consequences if they don’t take it.