WASHINGTON - The United States and Iran still have serious differences in restoring the 2015 Iran nuclear deal after six rounds of talks, a senior State Department official said on Thursday.

Washington and Tehran have had six rounds of indirect negotiations in Austria's capital Vienna since April aimed at reviving the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

"We still have serious differences that have not been bridged, serious differences with Iran over a host of issues, whether it's the nuclear steps that Iran needs to take ... the sanctions relief that the US would be offering or the sequence of steps that both sides will be taking," the official told reporters in a briefing call.

"Nothing is agreed until everything is agreed," added the official, who spoke on condition of anonymity.

The official suggested that a deal between the two sides remains possible, saying the US team would attend the seventh round of talks "sometime in the not too distant future."

"This process is not going to be open forever," the official cautioned. "We do have differences and if we can't bridge them in the foreseeable future, I think we're going to have to regroup and figure out how we move ahead."

The official noted that the election of Ebrahim Raisi as Iran's next president would not affect the US determination to try to reach a deal with Tehran.

Judiciary Chief Raisi on Satuday won Iran's presidential race by securing over 60 percent of votes, and he will assume office in August.

The president-elect on Monday urged the United States to lift "all unjust sanctions" against Iran and ruled out a meeting with President Joe Biden in his first press conference held after the election.

The US government withdrew from the JCPOA in May 2018 and unilaterally re-imposed sanctions on Iran. In response, Iran gradually suspended parts of its JCPOA commitments from May 2019.