WASHINGTON (REUTERS, AFP) - The chief US negotiator in Iran nuclear talks, Wendy Sherman, will leave the State Department shortly after the June 30 deadline for the talks between Teheran and six major powers to reach agreement, the New York Times said on Wednesday.

Ms Sherman, 65, made her intentions known before leaving for Vienna and Geneva for a last round of talks aimed at securing curbs on Iran's nuclear programme in return for lifting sanctions that have crippled the country's economy, the newspaper said.

"It's been two long years," the Times quoted Ms 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.

Ms 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 US 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.

United States 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."

"She's been the leader of a group that has now spread across many agencies and grown considerably since the negotiations started two years ago," William J Burns, Ms Sherman's predecessor, was quoted as saying by the Times.

Meanwhile, some officials warned Wednesday the negotiations may go beyond the June 30 deadline even as Mr Kerry prepares to once again huddle with his Iranian counterpart Mohammad Javad Zarif in Geneva for what is expected to be a final series of meetings on Saturday.

But Iranian negotiator Abbas Araghchi, quoted by state news agency IRNA, said Wednesday the two sides were "not bound by the schedule" agreed on April 2. "We are not at the point where we can say that negotiations will be completed quickly - they will continue until the deadline and could continue beyond that," he said.