Days before the start of crucial talks with world powers, Iran appears to be simultaneously hitting the gas and brake pedals on its nuclear program, speeding up production of enriched uranium while limiting its stockpile of the type of fuel that could be easily converted for use in atomic bombs, U.N. officials said in a new report.

International inspectors who visited Iran’s nuclear facilities last month confirmed that Tehran has begun installing hundreds of second-generation centrifuges that could dramatically increase its capacity to produce enriched uranium, the International Atomic Energy Agency said in a report leaked to journalists Thursday.

The new IR-2m machines are four times as productive as Iran’s existing centrifuges, which convert uranium gas into the enriched fuel that can be used in civilian power plants or nuclear weapons, intelligence officials and nuclear experts say.

Iran told the IAEA last month that it intended to install the more powerful machines, prompting warnings from Western governments that Tehran is moving rapidly toward a nuclear-weapons capability. On Thursday, the Obama administration said Iran’s latest advance, while expected, was provocative.

“The fact remains that the installation of new advanced centrifuges would be a further escalation, and a continuing violation of Iran’s obligations,” State Department spokeswoman Victoria Nuland told reporters.

But IAEA inspectors also reported new evidence that Iran is deliberately capping its uranium stockpile, perhaps to ease Western anxieties ahead of nuclear talks that are scheduled to begin next week in Kazakhstan. The nuclear watchdog noted that Iran had resumed converting some of its enriched uranium into metal fuel plates, rendering the material all but useless for weapons purposes.

Significantly, the type of uranium being converted to metal is enriched to 20 percent purity. It is considered particularly sensitive because it can be easily converted to weapons-grade fuel. Iran has, so far, kept its stockpile of this form of uranium below 550 pounds, the amount that would be needed, in theory, to make a single nuclear bomb.

In another possible sign of deliberate slowing, IAEA inspectors confirmed that Iran still has not begun operating hundreds of centrifuges installed recently at its newest uranium-enrichment plant, a facility built inside a mountain near the city of Qom.

Some Western officials and diplomats interpreted Iran’s mixed signals as a negotiating ploy ahead of next week’s nuclear talks. On Tuesday, Iranian officials are scheduled to begin negotiations in Almaty, Kazakhstan, with representatives of the United States and five other world powers on proposed limits to Iran’s nuclear program.

“We’ve seen this before, that in advance of diplomatic rounds there are new announcements of activity,” Nuland said. “It doesn’t make it any easier to get where we want to go.”

The six-party negotiating bloc, known as the P5-plus-1, is expected to pressure Iran to freeze production of 20 percent-enriched uranium in exchange for easing economic sanctions that have battered the Iranian economy. France, another of the P5-plus-1 powers, said Thursday that diplomats were preparing an updated offer to entice Iran to agree to cuts.

“We want a genuine exchange that will lead to concrete results,” a French Foreign Ministry spokesman told reporters in Paris.

The IAEA report hinted at future conflicts over Iran’s nuclear program, even if a deal is reached in Almaty. The agency reported that Iran plans to begin using an existing medical research reactor to test fuel rods for a heavy water reactor that Iran has been building in Arak for years. Western officials say the reactor could become operational next year. When it does, Iran would have the ability to produce plutonium as well as enriched uranium, providing it with a second pathway to a possible nuclear weapon.