"We do see this as a constructive and certainly welcome step," State Department spokesman Ned Price told reporters, while noting that the United States has not directly participated.

"It is a potentially useful step as we seek to determine what it is that the Iranians are prepared to do to return to compliance with the stringent limitations under the 2015 deal, and as a result what we might need to do to return to compliance ourselves."

European allies of the United States along with Russia and China are meeting in Vienna with Iran in the most forceful bid yet to revive the nuclear agreement from which former president Donald Trump withdrew the United States.

With Iran refusing to meet the United States directly, US envoy Rob Malley is at a separate hotel with European diplomats planning to shuttle messages within the city.

Iran wants Biden first to lift sweeping sanctions imposed by Trump when he left the agreement in 2018 and launched a "maximum pressure" campaign to fight Tehran on multiple fronts.