Sherman made her intentions known before leaving for Vienna and Geneva for a last round of talks aimed at securing curbs on Iran’s nuclear program in return for lifting sanctions that have crippled the country’s economy, the newspaper said.

“It’s been two long years,” the Times quoted Sherman as saying, referring to the protracted and intensely complex discussions with the Iranians that began with secret talks between the two long-time foes.

Sherman, who is under secretary of state for political affairs at the State Department and who has white, close-cropped hair, has stood out as one of the few women in a sea of men in dark suits at most of the negotiating sessions.

Her departure will leave a big gap in the U.S. team, which will have to manage the arrangements for rolling back sanctions and monitoring Iran’s compliance should a final deal be struck next month.

Secretary of State John Kerry said in a statement to CNN: “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.”