…American officials say that they have been tracking the covert project for years, but that Mr. Obama decided to make public the American findings after Iran discovered, in recent weeks, that Western intelligence agencies had breached the secrecy surrounding the project. On Monday, Iran wrote a brief, cryptic letter to the International Atomic Energy Agency, saying that it now had a “pilot plant” under construction, whose existence it had never before revealed.

But President Mahmoud Ahmadinejad said nothing about the plant during his visit this week to the United Nations, where he repeated his contention that Iran had cooperated fully with inspectors, and that allegations of a nuclear weapons program are fabrications.

The newly discovered enrichment plant is not yet in operation, American officials said, but could be next year.

Mr. Obama’s planned announcement with Mr. Brown and Mr. Sarkozy will likely overshadow the meeting of the Group of 20 industrial nations, whose leaders have gathered to plan the next steps in combating the global financial crisis. Instead, here and during the opening of the United Nations in New York, senior officials from several of the countries were pulled aside for briefings on the new intelligence and strategy sessions about the first direct talks with Iran in 30 years that will include the United States.