WASHINGTON (Reuters) - U.S. Secretary of State John Kerry said on Thursday the implementation of a nuclear deal agreed between Iran and six world powers was only days away, allowing tens of billions of dollars in sanctions against Iran to be lifted.

There is no date set yet for "implementation day" of the so-called Joint Comprehensive Plan of Action agreed on July 14 in which Tehran agreed to shrink its nuclear program in exchange for some sanctions relief.

Outlining foreign policy milestones of the past year, Kerry pointed to the Joint Comprehensive Plan of Action "from which we are days away from implementation, if all goes well."

He said he discussed implementation of the nuclear deal with Iranian Foreign Minister Javad Zarif during a phone call on Thursday. They also talked about tensions between Iran and Saudi Arabia triggered by the execution by the Saudis of a Shi'ite cleric, he said.

"The foreign minister made it clear to me they intend to complete their obligations with respect to implementation day as rapidly as possible," said Kerry told reporters.

"We are currently engaged ourselves in making certain that we're prepared to move on that day. And I think it could come -- without being specific -- sooner rather than later."

Kerry said the United States would continue to ensure that Iran lives up to its commitments under the nuclear deal and press for the release of American prisoners "that have been unjustly detained" by Iran.

Senior Iranian officials have dismissed speculation that Iran is considering a prisoner exchange with the United States. Among Americans held by Iran is Washington Post correspondent, Jason Rezaian, who was arrested in July 2014.

Others include Christian pastor Saeed Abedini and Amir Hekmati, a former sergeant in the U.S. Marine Corps. Robert Levinson, a private investigator, disappeared in Iran in 2007. Iran has claimed that the United States holds some 16 Iranians for bypassing sanctions and around 60 prisoners for other crimes.

(Reporting by Lesley Wroughton)