EU foreign policy chief Federica Mogherini was due in Tehran Tuesday for talks on implementing this month's historic nuclear deal between Iran and the major powers.
As the European Union's high representative, Mogherini was a lead negotiator for the six powers in the long-running diplomacy that culminated in the July 14 agreement.
She flies in from talks in Iran's regional rival Saudi Arabia, whose leaders told her of their deep misgivings about Tehran's intentions.
Riyadh has not publicly condemned the agreement but the deal has come under furious attack by Israel and Republican lawmakers in the US Congress.
Mogherini will meet Iran's Foreign Minister Mohammad Javad Zarif in Tehran before a joint news conference at 1130 GMT.
Her visit "will provide an opportunity for exchanges on the implementation of the agreement" on which she "will continue to play a coordinating role," her office said.
Under the deal, Iran must take measures at its nuclear facilities to satisfy UN monitors of their peaceful nature.
In return, the powers must finalise the mechanism for the lifting of UN and Western sanctions.
The deal has been strongly welcomed in Tehran with officials lining up to back the government's success in securing relief from crippling sanctions in exchange for curbs on some but not all of its nuclear activities.
As the agreement goes under a potentially bruising review in the US Congress, President Barack Obama has defended it as the best way to avoid nuclear proliferation and another possible conflict in the Middle East.
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Iran has always denied seeking an atomic weapon but the deal has not put a stop to the accusations of its arch critics that the bomb is its eventual aim.
- Iran, Saudi at loggerheads -
Obama has sought to head off the criticism by stressing that the deal does not preclude military action if Iran welches on its side of the bargain, comments that have met with a frosty reception in Tehran.
The nuclear deal has further strained Iran's relations with Saudi Arabia, already troubled by Saudi-led military intervention against fellow Shiites in Yemen.
On Monday, Saudi Foreign Minister Adel al-Jubeir denounced "aggressive statements" by Tehran, in an apparent reference to charges by his Iranian counterpart that "some countries... want conflict and war in this region."
Zarif did not name Saudi Arabia but his remarks during a visit to Kuwait on Sunday came after Bahrain, a Sunni-ruled kingdom with a Shiite majority, accused two men of smuggling weapons into its territory from Iran.
Zarif dismissed the claims as a "baseless" attempt to prevent any progress or cooperation between Iran and Gulf states.
Jubeir said he brought up the "unacceptable" Iranian remarks during Monday's talks with Mogherini, in which he was also briefed about safeguards included in this month's nuclear agreement.
Mogherini's trips to Riyadh and Tehran are the latest in a string of regional visits by top Western officials following the Iran accord.
French Foreign Minister Laurent Fabius will travel to Tehran on Wednesday. How he will be received "will for us be an evaluation of Iran's behaviour," President Francois Hollande said on Monday.