But he said there is an opportunity to reach a verifiable deal with Tehran to end Iran's nuclear weaponry ambitions, and piling on additional sanctions now would be the wrong strategy.
"The sanctions that we put in place helped make this opportunity possible," he said. "But let me be clear: If this Congress sends me a new sanctions bill now that threatens to derail these talks, I will veto it. For the sake of our national security, we must give diplomacy a chance to succeed."
Notable deaths of 2014 [PHOTOS]