Iran nuclear deal 'days away' from implementation: Kerry

US Secretary of State John Kerry has said that the implementation of the Iran nuclear deal may be only "days away," with Tehran on track to meet its pledge to put a bomb beyond its reach.

Kerry said on Thursday that he had spoken to Foreign Minister Mohammad Javad Zarif, who promised that Iran would live up to its promises, and Kerry added that the US plans to gradually lift its nuclear-related sanctions against Tehran.

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

"And 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 Iran had shipped its stock of low-enriched uranium to Russia, making Iran unable to achieve nuclear capabilities for at least a year even if it changed its mind.

"With that, Iran literally shipped out its capacity currently to build a nuclear weapon," he said.

"We went from two months of potential breakdown time, two to three months to nine months. And in the next days we will meet our task of being more than a year of breakout time."

Under the plan, agreed in July between Iran and the US, China, Russia, France, UK and Germany - Tehran will almost completely halt its nuclear programme in exchange for sanctions relief.

The United States and European Union have begun preparations to drop their nuclear-related sanctions, but will not do so until "implementation day", after the UN nuclear watchdog IAEA verifies that Iran has held its end of the bargain.