– Iran is expected to make an offer Tuesday to scale back its effort to enrich uranium, a move that a year ago would have been a significant concession to the West. But Iran’s nuclear abilities have advanced so far since then that experts say it will take far more than that to assure the West that Tehran does not have the capacity to quickly produce a nuclear weapon.

With thousands of advanced centrifuges spinning and Iranian engineers working on a plant that will produce plutonium — which also can be used in a weapon — Iran’s program presents a daunting challenge for negotiators determined to roll back its nuclear activities.

Both sides enter the nuclear talks that begin here Tuesday with inherent strengths and weaknesses. Iran walks in with a nuclear program that cannot easily be turned back, while the West has imposed sanctions that have crippled the Iranian economy.

And if Iran is going to maintain the right to enrich uranium to even low levels, as it continues to insists it must, the West would surely demand highly intrusive inspections — far more than Iran has tolerated in the past. How these matters are resolved will go far in deciding the success or failure of the talks.

In 2003, Iran had a relative handful of relatively unsophisticated centrifuges. Today, it has at least 19,000, and 1,000 of those are highly advanced.

Those have been installed but are not yet being used to enrich uranium.

That is more than enough, experts say, to transform low-enriched uranium from the 3 percent to 5 percent range to weapons grade in a few months.

That would provide Iran with a so-called breakout capability that is unacceptable to the West and Israel, even if, as expected, Iran proposes a moratorium on enrichment to 20 percent.

“Ending production of 20 percent enriched uranium is not sufficient to prevent breakout, because Iran can produce nuclear weapons using low-enriched uranium and a large number of centrifuge machines,” said Gary Samore, a senior aide on nonproliferation on the National Security Council in President Obama’s first term.

In addition, Tehran is nearing completion of a heavy-water reactor that would be capable of producing plutonium for nuclear bombs, another factor that Western experts say argues for far broader constraints.

The talks in Geneva are the first between Iran and the United States and five other world powers since the election of Iran’s new president, Hassan Rowhani, who took office in August and who has made a priority of easing the crippling sanctions imposed on Iran over its nuclear activities.

Diplomatic opening

A series of conciliatory messages and speeches from Rowhani and other Iranian officials — capped by a phone call to the Iranian president from Obama last month — has helped foster the most promising atmosphere for negotiations since 2003, when Rowhani was Tehran’s lead nuclear negotiator.

A senior American official said Monday that the United States was heartened by the change of tone in Tehran and believed that Rowhani’s election signals a sincere intention by Iran to chart “a more moderate course.”

But the official also said that the United States and its partners were still waiting to see if Iran would take concrete steps to constrain the pace and scope of its nuclear program, limit its growing stockpile of enriched uranium and be more open about its nuclear activities.

American officials have said that they are prepared to reciprocate, and the U.S. delegation here includes a senior expert on economic sanctions: Adam Szubin, the director of the Office of Foreign Assets Control at the Treasury Department.

Sanctions

But the United States is reluctant to withdraw the most effective measures, especially sanctions that have cut off Iran from the international banking system, until the main issues are solved.

Any easing of sanctions would be “proportional to what Iran puts on the table,” said the senior U.S. official, who added that it was likely that the Iranians would “disagree about what is proportionate.”

Another potential obstacle is Iran’s insistence that its right to enrich uranium be acknowledged now as part of the negotiations under which it would accept constraints on its nuclear activity.

The senior U.S. official said that Iran had a right to a civilian nuclear energy program and that the United States was now prepared to talk about a “comprehensive” solution. But the official would not say whether Iran should be allowed to produce enriched uranium at home or limited to acquiring nuclear fuel from other nations.

“We are prepared to talk about what President Obama said in his address at the U.N.,” the senior official said. “That he respects the rights of the Iranian people to access a peaceful nuclear program. What that is is a matter of discussion.”

Ray Takeyh, a former State Department expert on Iran, underscored the obstacles to a quick breakthrough.

“Both sides are victims of their success today,” Takeyh said.

“Iranians have a mature nuclear program that they are reluctant to trade. The Americans have a substantial sanctions regime that they are averse to dismantling for anything but measurable Iranian concessions.”