A deal between Iran and six major powers, intended to pave the way to a solution to a long standoff over Tehran's nuclear ambitions will come into force on January 20, officials say.

The Iranian Deputy Foreign Minister made the announcement on television.

An Iranian official said that after the interim deal takes effect, Tehran and world powers will start negotiating a final settlement… In hopes of settling their differences about activity the West suspects is aimed at obtaining a nuclear weapons capability.

U.S. Secretary of State, John Kerry said the next phase would be "very difficult".

"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...but they are the best chance we have to be able to resolve critical national security issues peacefully and durably."

President Obama said the US and other nations would begin to give Iran "modest relief" on economic sanctions as long as the Islamic Republic lived up to its end of the agreement.

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