Iranian President Hassan Rouhani speaks during a parliament session in Tehran on Nov. 10. (Ebrahim Noroozi/AP)

Iran appears to have dramatically slowed work on its atomic energy program since the summer, U.N. officials said Thursday. The report could add momentum to diplomatic efforts to resolve a decade-old dispute over Iranian nuclear activities.

The report by the International Atomic Energy Agency said Iran all but halted the installation of new centrifuges at its uranium enrichment plants beginning in August, the same month that moderate cleric Hassan Rouhani was sworn in as president. Work on a controversial nuclear reactor also slowed, the U.N. watchdog agency said. Iran continued producing low-enriched uranium, but at a slightly reduced rate, it said.

The findings provided a boost to the Obama administration, which has joined five other major powers in seeking to negotiate a deal on permanent limits to Iran’s nuclear program. The report suggests that Iran has been unilaterally implementing key parts of a nuclear “freeze” that Western nations have been pursuing during nuclear talks.

The negotiations are scheduled to resume next week in Geneva.

News of the apparent slowdown came as President Obama repeated his appeal to Congress to delay consideration of further economic sanctions on Iran while diplomatic efforts are underway.

President Obama said on Thursday that if the U.S. is truly "serious about pursuing diplomacy" with Iran, then it did not need to place any more sanctions over the country's nuclear program. (Reuters)

“Let’s test how willing they are to actually resolve this diplomatically and peacefully,” Obama said Thursday at a news conference. “We will have lost nothing if, at the end of the day, it turns out that they are not prepared to provide the international community the hard proof and assurances necessary for us to know that they’re not pursuing a nuclear weapon.”

The IAEA report is a periodic snapshot of Iran’s nuclear program by the only group of outside experts allowed regular access to the country’s nuclear energy facilities. In recent years, the U.N. agency has documented a dramatic expansion in Iran’s capability to make enriched uranium, with the addition of thousands of centrifuges at its main enrichment plant in Natanz as well as a smaller, underground facility known as Fordow.

Iran also had been seen making steady progress on a partially constructed nuclear reactor near the city of Arak. The heavy-water reactor is ostensibly intended for medical research and isotope production, but it has raised proliferation concerns because its spent fuel can be reprocessed to extract plutonium, which can be used in nuclear weapons.

The new report shows a sharp drop in activity at each of the sites, starting around the time that Rouhani took office. At Natanz, where Iran had been adding centrifuges at a rate of 600 a month, only four new machines have been put in place since the summer, the IAEA report said. No new centrifuges were installed at Fordow, and work on new reactor components at the Arak plant appears frozen, the report said.

“It shows that they’re not adding capacity, at least at Natanz and Fordow,” said David Albright, a former U.N. nuclear inspector and president of the Institute for Science and International Security, a Washington-based research group.

But Albright noted that Iran has not halted uranium enrichment, and its total stockpile of low-enriched fuel continued to rise. Iran’s inventory of a more-enriched form of uranium — called 20 percent-enriched uranium — grew to 432 pounds, an increase of 10 pounds but still less than the amount theoretically needed to make single bomb.

For the moment, Iran’s voluntary restraint “would appear to be a sign of good faith in the ongoing negotiations,” said Daryl Kimball, executive director of the Washington-based Arms Control Association. “However, the IAEA report also shows that Iran is on the verge of several technical milestones that would enable it to quickly increase its uranium enrichment capacity.”

Iran produces low-enriched uranium used as fuel in nuclear power plants and some research reactors. With additional enrichment, the same material could be converted to weapons-grade uranium used in nuclear bombs. The United States and other Western powers have been pressing Iran during nuclear talks to accept restrictions that would make it virtually impossible for it to make weapons-grade fissile material without being detected. As a proposed first step toward a deal, Iran has been offered modest relief from some economic sanctions if it agrees to freeze key parts of its nuclear program.

Israeli officials and several prominent members of Congress have argued that economic pressure on Iran should be further increased until it agrees to completely dismantle its nuclear infrastructure — something Iran insists it will never do.

Obama, in the news conference Thursday, said the proposed agreement would provide the best guarantee against both a nuclear-armed Iran and another war in the Middle East.

“We’re serious about trying to resolve this diplomatically, because no matter how good our military is, military options are always messy, are always difficult, always have unintended consequences,” Obama said.

Obama claimed partial credit for Iran’s willingness to negotiate, noting that his administration, working with Congress and international allies, had imposed the tough economic sanctions that have devastated Iran’s economy.

A new report Thursday showed Iran’s oil imports falling further in recent weeks, plummeting 45 percent since September to the lowest level in nearly two years.

Steven Mufson contributed to this report.