President Joe Biden seeks a return to the 2015 deal, known as the Joint Comprehensive Plan of Action, but has not changed his position that Iran must abide by it first.

"If Iran comes back into full compliance with its obligations under the JCPOA, the United States would do the same," the State Department's new spokesman, Ned Price, told reporters.

The administration will be "consulting with our allies, consulting with our partners, consulting with Congress before we're reaching the point where we're going to engage directly with the Iranians and (be) willing to entertain any sort of proposal," Price said.

Iran has insisted that the United States first lift sanctions imposed by former President Donald Trump before Tehran pulls back nuclear steps it took to protest his pressure campaign.

Offering a way out of the diplomatic stalemate, Iranian Foreign Minister Mohammad Javad Zarif in an interview broadcast Monday proposed that the European Union coordinate to "synchronize" actions by the United States and Iran.