U.S. negotiators are said to have given up ground on demands that Iran be forced to disclose the full range of its nuclear activities at the outset of a nuclear deal, a concession experts say would gut the verification the Obama administration has vowed would stand as the crux of a deal with Iran.

Until recently, the Obama administration had maintained that it would guarantee oversight on Tehran’s program well into the future, and that it would take the necessary steps to ensure that oversight would be effective. The issue has now emerged as a key sticking point in the talks.

Concern from sources familiar with U.S. concessions in the talks comes amid reports that Iran could be permitted to continue running nuclear centrifuges at an underground site once suspected of housing illicit activities.

This type of concession would allow Iran to continue work related to its nuclear weapons program, even under the eye of international inspectors. If Iran removes inspectors—as it has in the past—it would be left with a nuclear infrastructure immune from a strike by Western forces.