During his two-day visit, Enrique Mora will meet with the US special envoy for Iran, Rob Malley, as well as the top White House official on the Middle East, Brett McGurk, and the number three US diplomat, Victoria Nuland.

On Wednesday, he is also due to take part in the first session of the high-level dialogue between the United States and the EU on Russia, amid the war sparked by its invasion of Ukraine over a month ago.

Mora's visit comes after he met with Iranian officials on Sunday in Tehran to try to close the gaps remaining in the long and complex negotiations to save the 2015 agreement on Iran's nuclear program.

EU foreign policy chief Josep Borrell had said over the weekend that an agreement could come in "a matter of days," but took a made a more downbeat assessment Monday after new obstacles emerged.

The Joint Comprehensive Plan of Action (JCPOA) gave Iran much-needed sanctions relief in exchange for curbs on its nuclear program that would guarantee it could not develop a nuclear weapon -- something it has always denied seeking.

But it fell apart in 2018, when then-president Donald Trump withdrew the United States and reimposed crippling economic sanctions.

Iran, in response, began rolling back on most of its commitments under the accord.

Current US President Joe Biden took steps shortly after taking office to revive the deal, promising an easing of punishing sanctions in exchange for restoring JCPOA controls.

Indirect negotiations between the rival states, which have been underway in Vienna for nearly a year, have overcome repeated disagreements but outstanding issues remain.

Among them is a Tehran demand that the Islamic Revolutionary Guard Corps (IRGC), the ideological arm of Iran's military, be removed from a US terror blacklist, which Washington has resisted.

On Sunday at the Doha Forum in Qatar, Malley said: "The IRGC will remain sanctioned under US law and our perception of the IRGC will remain."

According to experts, however, this does not mean the United States would refuse to remove the Guards from the blacklist, as IRGC leaders would still remain under other sanctions.