WASHINGTON  A dissident group that had previously revealed the existence of several hidden nuclear sites in Iran claimed Thursday that it had evidence that the country was building another secret uranium enrichment plant.

The group, the People’s Mujahedeen of Iran, showed satellite photographs of an extensive tunnel-digging operation near a military garrison northwest of Tehran. But the group had no pictures of the interior and no evidence to back up its claim that the site was intended to hold several thousand centrifuges, the machines used to enrich nuclear fuel for power production or weapons. The Obama administration, which publicly revealed evidence a year ago of a hidden nuclear facility near the holy city of Qum, reacted cautiously to the group’s announcement.

In recent months officials have said they had no evidence of another enrichment facility, though they have expressed suspicions about a number of deep tunnels built into hillsides or mountains. One United States government official said the evidence from the People’s Mujahedeen, which was turned over to American officials this week, would require careful examination.

A new enrichment plant, if it existed, would heighten suspicions that Iran was trying to evade international inspectors and find another way to produce fuel usable in a bomb. But there are other possible explanations for an underground facility, including a location to store conventional weapons.