Western powers suspect Iran has been trying to develop the ability to manufacture nuclear weapons. Iran says its program is aimed purely at generating electricity and fueling civilian atomic research.
Kerry stressed the United States and its allies will work closely with UN nuclear inspectors to ensure that the terms of this deal are met.
"As this agreement takes effect, we will be extraordinarily vigilant in our verification and monitoring of Iran's actions," said Kerry.
He added the international community is clear-eyed about the greater challenges of a more comprehensive agreement restricting Iran's nuclear program.
"We are very clear about what will be required in order to be able to guarantee to the international community that this is a peaceful program. The negotiations will be very difficult, but they are the best chance that we have to be able to resolve this critical national security issue peacefully and durably," said Kerry.
In a written statement, President Barack Obama said the United States and other nations will begin to give Iran "modest relief" on economic sanctions, as long as Tehran lives up to its end of the agreement.
Obama said he will veto any new sanctions legislation passed by the U.S. Congress during talks on a long-term deal with Iran, but he said Washington will be ready to increase its sanctions if Iran fails to abide by the agreement.