The US on Thursday called on recently-seated Iranian President Ebrahim Raeisi to return to negotiations aimed at bringing Washington and Tehran back into compliance with the 2015 nuclear accord.

State Department spokesman Ned Price urged Raeisi's government "to return to diplomacy" after the sixth round of indirect negotiations concluded in June without follow-on talks taking place.

"We urge Iran to return to the negotiations soon so that we can seek to conclude our work," Price told reporters.

"The opportunity to achieve a mutual return to compliance with the JCPOA won’t last forever. The longer this goes on, the advantages to our national security that would be accrued by a mutual return to compliance, will start to chip away by the advancements that Iran is able to make while the shackles are, at present, removed from its nuclear program," he added.

The sixth round of talks concluded with both sides voicing views that were sharply at odds with each other about their status.

Iran maintained that the talks concluded with each party agreeing on a clear text on all issues while Washington said serious differences remained.

Raeisi's administration is reportedly mulling a new format and approach to negotiations.

In his speech after taking the oath of office, Raeisi said sanctions imposed on Iran "must be lifted," while welcoming "any diplomatic initiative" to that end, in an apparent green light to ongoing negotiations between Iran and the West on salvaging the deal.

Former US President Donald Trump unilaterally withdrew Washington from the nuclear agreement with Iran in 2018 and went on to re-impose sanctions on Tehran lifted under the deal in a failed push to bring Iran back to the negotiating table for a more extensive agreement.

US President Joe Biden has since sought to pursue negotiations that would enable Washington and Tehran to resume full compliance with the pact.

The Joint Comprehensive Plan of Action, as the agreement is formally known, placed sweeping constraints on Iran's nuclear program and created a robust inspections regime in exchange for the lifting of biting international and US sanctions.