Iran is expanding the capacity of its controversial underground nuclear facility, a U.N. report said Friday, as its leaders move to increase production of a more purified form of enriched uranium in defiance of Western demands for a freeze.

U.N. inspectors who visited the plant near the city of Qom earlier this month saw hundreds of newly installed centrifuges amid steady progress in boosting the capability of the facility, which has come to symbolize international concerns about Iran’s possible pursuit of nuclear weapons.

They also discovered traces of a form of uranium that is closer to the kind needed to make weapons-grade fuel than the Iranians have previously acknowledged making. The particles were believed to have resulted from a technical glitch, but officials from the International Atomic Energy Agency were continuing to investigate the matter.

Evidence of the plant’s expansion is likely to add to worries about Iran’s nuclear ambitions, while enhancing the country’s bargaining position going into a new round of nuclear talks scheduled for June, weapons experts said.

“Iran is dealing itself more cards for the negotiations,” said Joshua Pollack, a government consultant on nuclear issues and a contributor to “The West is piling on sanctions while they’re adding more [centrifuges] underground. We’ll see who blinks first.”

The IAEA report, a summary of findings from the agency’s inspections inside Iran, documented a jump in the country’s overall production of enriched uranium, suggesting that the country is continuing to recover from a disastrous computer virus two years ago and other technical setbacks.

“The machines seem to be operating better, and overall they’re enriching more efficiently,” said David Albright, a former IAEA inspector and president of the Institute for Science and International Security.

The trace particles of a form of more highly enriched uranium were discovered during IAEA tests of environmental samples collected during a previous inspection at Qom in February. The particles were found to have been enriched to 27 percent purity. Although that is a level higher than Iran has previously acknowledged making, it is still well below the 90 percent level needed for nuclear weapons.

When pressed about the anomaly, Iran said a spike in enrichment levels could happen “for technical reasons outside the operator’s control,” the report said.

IAEA officials have taken additional samples while an investigation continues, though several nuclear experts asserted that the unusual particles could have resulted from ordinary fluctuations in the enrichment process.

Iran contends that it needs the enriched uranium to fuel nuclear power plants, while Western governments suspect Iran’s nuclear activities are a cover for a secret weapons program.