The Obama administration insisted before lawmakers that any deal with Iran would ensure for "perpetuity" that it could not develop nuclear weapons. A comprehensive accord would also see "phased, proportionate" relief from sanctions that have severely constrained Iran's economy, but such relief could be reversed should the Islamic republic violate any final deal, said deputy secretary of State. Several members of Congress have warned that the ongoing negotiations between world powers and Tehran would lead to a deal that would sunset after 10 years.