With another round of nuclear talks to begin this week between Washington and Tehran, the top U.S. diplomat said the obstacles to an agreement are largely political, rather than technical.
"There are clearly some differences that still rest on a technical judgment," Secretary of State John Kerry said in Sunday on CBS television's Face the Nation program. "But by and large, most of the differences now are political decisions ... to fulfill the promise of proving to the world that a program is peaceful."
Kerry will meet in Switzerland with Iranian counterpart Mohammed Javad Zarif in the next round of talks on the future of Iran's nuclear program.
Meanwhile, U.S. Senate Republican leader Mitch McConnell issued a statement against the talks, telling CNN President Barack Obama was on the cusp of a "very bad deal" with Tehran.
Watch Michael Bowman's report:
Kerry said he is unsure if an open letter sent by McConnell and 46 other Republican U.S. senators last week to Iranian leaders had jeopardized the future of the ongoing talks.
The group warned that the next U.S. president could revoke a deal at any time, a statement the secretary of state challenged.
The United States and its partners in the P5+1 group, Britain, China, France, Russia and Germany, have an end-of-March deadline to reach a framework deal for Iran to cut back its uranium enrichment program.
Tehran said economic sanctions must be lifted for a deal to be achieved; the country insists its nuclear program is strictly for civilian purposes.
U.S. lawmakers who oppose the deal say Iran cannot be trusted.