A deal to limit Iran's nuclear program in exchange for limited sanctions relief is set take effect January 20.
Officials from both sides confirmed the start date Sunday after experts spent weeks discussing how to implement the six-month pact they first agreed to in November.
Iran will limit its uranium enrichment to five percent and reduce its stock of higher-enriched uranium, while also allowing United Nations inspectors to access nuclear facilities.
In exchange, the United States and European Union will ease sanctions that have squeezed Iran's economy.
Iran and the six world powers, which include the U.S., Britain, China, France, Russia and Germany, will use the six months to continue negotiations on a long-lasting deal to address fears that Iran is seeking to build nuclear weapons.
U.S. Secretary of State John Kerry said reaching a comprehensive agreement will be tough, but that the negotiations are the best way to resolve the issue "peacefully and durably."
"We are very clear-eyed about the even greater challenges that we face negotiating a comprehensive agreement. We understand it is going to be a tough negotiation and 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," said Kerry.
President Barack Obama said the United States and others will give Iran "modest relief" on economic sanctions as long as Tehran lives up to its end of the agreement.
Obama also said that he will veto any new sanctions legislation passed by the U.S. Congress during talks on a long-term deal with Iran, but that the U.S. will be ready to increase its sanctions if Iran fails to abide by the agreement.
The West has accused Iran of working toward building a nuclear weapon. Iran has long maintained that its nuclear program is purely for generating electricity and other civilian purposes.