The "tenacious" top US negotiator in talks with Iran on curtailing its nuclear program will leave her post after a June 30 deadline for an agreement, a senior State Department official said Thursday.

Secretary of State John Kerry was effusive in his praise of Wendy Sherman, 65, describing her as "indefatigable, tough and creative" and saying she had played a central role in reining in Tehran's nuclear ambitions.

Sherman will leave the post of undersecretary of state for policy when the Iran talks are completed, the senior State Department official told AFP.

"Wendy has been an absolutely critical member of my team, most notably in her work spearheading the nuclear negotiations with Iran, but on nearly every important issue that has crossed my desk," said Kerry.

Sherman, the number three at the US State Department, broke the news to staffers on Wednesday then left on her latest trip for talks with Iran in Switzerland, The New York Times said.

"It's been two long years," the Times quoted Sherman as saying.

It added that, with the departure of Sherman, all the top US officials who have negotiated with Iran over that span will have left President Barack Obama's administration.

"I will miss her collaboration and her counsel enormously," said Kerry, also lauding "her calm in the storm."

"Wendy's tenacity and skill, and her close relations and collaboration with her P5+1 and EU counterparts were instrumental in enabling us to reach the interim agreement that halted the progress of Iran's nuclear program for the first time in almost a decade and made our nation, our partners and allies -- and our world -- safer," Kerry added.

Kerry on Saturday will once again meet Iranian counterpart Mohammad Javad Zarif in Geneva, after weeks of behind-the-scenes technical discussions in Vienna seeking to narrow the gaps on what would be an unprecedented deal on curtailing Iran's nuclear program.

June 30 is the deadline for a comprehensive agreement.

National Security Advisor Susan Rice was also fulsome in her praise of Sherman.

"In Wendy Sherman, couldn't have had a better partner. Lots of work left for her, including with Iran, but wishing her well on the next chapter," Rice tweeted.