Over a month after Richard Nephew left the US negotiating team in Vienna, he has published a series of tweets to elaborate on the reasons behind his move.

Nephew, who is known as the architect of Washington’s sanctions on Iran, was serving as the deputy to US especial envoy to Iran Robert Malley until December 6, when he left the US negotiating team along with two others, over what was reported as differences with Malley at the time.

In a series of tweets on Tuesday, Nephew said he stepped down from his post “due to a sincere difference of opinion concerning policy”, adding that he no longer serves in the State Department since last week and intends to return to academia.

“Although my views and record have been and continue to be mischaracterized by quite a few people, I do not intend to convey any further details at this time or in public, given the ongoing nature of discussions in Vienna”, he added.

Advocating a tougher posture, Nephew, along with two other US negotiators, left the team because the Biden administration failed to achieve a more assertive approach in the talks, the Wall Street Journal reported.

Nephew, who is a non-resident Senior Fellow at the Brookings Institution, published a book about the sanctions in 2017, titled The Art of Sanctions: A View from the Field.