US Secretary of State John Kerry said Thursday 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 told reporters he had spoken to Foreign Minister Mohammad Javad Zarif, who insisted Iran would live up to its promises, and the secretary said the United States would be ready to begin lifting 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."

Earlier, Kerry had hailed the deal's "Joint Comprehensive Plan of Action, from which we are days away from implementation, if all goes well."

Kerry said Iran had shipped its stock of low-enriched uranium to Russia and that additional steps Iran was taking would leave it at least a year from achieving a nuclear capability 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 JCPOA plan, agreed in July between Iran and the P5+1 group of world powers, Tehran agreed to rein in its nuclear program -- which Western capitals alleged was aimed at building a weapon -- 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 unheld its end of the deal.