The diplomat said that the Iranians have paved over a large part of the area surrounding a building where the IAEA suspects the Iranians were conducting nuclear experiments.

CNN: Story highlights

The U.S. believes sub-critical experiments are permitted but hydro-nuclear tests are not.

CENTERFORSECURITYPOLICY: Center for Security Policy | Politics and the Test Ban Treaty, By George F. Will, Newsweek, 20 October 1999

That is, the assessments must be grounded on existing nuclear test data, nonnuclear tests, fundamental science experiments, and simulations using validated computer models.

CENTERFORSECURITYPOLICY: US Nuclear Stockpile