By Jon Leyne
BBC Tehran correspondent
Iran is under pressure to provide reassurances about its nuclear facilities
As Iran goes into a new round of talks over its nuclear programme, the latest revelation about a new uranium enrichment plant will put it on the defensive.
President Ahmadinejad appeared a little taken aback, when he was challenged at an interview with Time Magazine, shortly before President Obama spoke.
He insisted that Iran had not been engaging in secrecy.
Then he warned Mr Obama that Washington would have to apologise to Iran if it raised the issue.
Iran argues that it is not obliged to declare nuclear facilities until 180 days before they become operational.
But Western sources say they believe Iran has been working on the new enrichment plant since 2006.
They say that would be in clear defiance of UN resolutions calling Iran to cease enrichment-related activity.
Above all, this latest revelation will further damage Iran's credibility, and its argument that the nuclear programme is peaceful and open.
Instead, it plays into fears that Iran might have a second, secret, nuclear programme, running in parallel with the declared facilities that have been opened to inspection by UN nuclear watchdogs.
Yet, despite the international condemnation, in some ways nothing has changed.
Iran begins a round of new talks on Thursday with the so-called G5+1 - the group of major powers who are negotiating with Tehran on the issue.
Iran will be under new pressure, in the Geneva talks, to provide answers and assurances.
But the Iranian government's strategy is likely to be one of continued delay, trying to talk out the issue, while it continues with the enrichment programme.
For the West, the challenge is to keep up the pressure, and build up momentum towards a possible new round of sanctions.
Russia seems to have lifted its veto on new sanctions.
But, despite the day's dramatic developments, the long slow process of diplomacy continues to grind on.